Illinois Form IL-1120-ST is the Small Business Corporation Replacement Tax Return that every S corporation doing business in Illinois must file each year with the Illinois Department of Revenue to report income, compute the 1.5% personal property replacement tax, pass income through to shareholders on Schedule K-1-P, and, when elected, pay the 4.95% Pass-through Entity (PTE) tax under 35 ILCS 5/201(p). The form is the Illinois mirror of federal Form 1120-S, and skipping it (or filing it wrong) triggers automatic late-filing penalties under 35 ILCS 735/3-3 plus interest that compounds daily until the balance is paid.
The Illinois Department of Revenue processes more than 90,000 IL-1120-ST returns each year, and internal IDOR review data shows that roughly one in five paper-filed returns gets flagged for a math error, missing schedule, or shareholder mismatch on Schedule B. Filing electronically through MyTax Illinois cuts that error rate dramatically and gives you a same-day confirmation number you can rely on if the state ever loses your return.
- 📋 The exact line-by-line walkthrough of every step on the 2025 IL-1120-ST, including Steps 1 through 8 and the most-missed boxes
- 💰 How the 1.5% replacement tax and the optional 4.95% PTE tax actually work, and when the election saves your shareholders federal tax
- 🧾 Three full filled-out scenarios using named filers so you can match your situation to a real example
- ⏰ Deadlines, the automatic 7-month extension, late-filing penalties, and the proof-of-filing you must keep
- 🚫 The ten most common mistakes that trigger IDOR notices, and how to avoid each one before you click submit
What the Form Is and Who Must File It
Illinois Form IL-1120-ST is the annual income and replacement tax return for small business corporations, which is Illinois’ label for federal S corporations. The form is authorized under the Illinois Income Tax Act, 35 ILCS 5/502, and the current version carries a revision date printed in the lower-left corner of page 1 (the 2025 tax-year form is marked IL-1120-ST (R-12/25)).
Every S corporation that is organized in Illinois, qualified to do business in Illinois, or that derives any income from Illinois sources must file. That includes S corporations with zero Illinois income, because the replacement tax computation and the shareholder pass-through schedules are still required. If your federal S election is in place for the year, you file IL-1120-ST in Illinois, never the C-corp form IL-1120.
The plain-English purpose of the form is to (1) compute the 1.5% replacement tax that S corporations pay on Illinois base income under 35 ILCS 5/201(c), (2) pass income, deductions, and credits through to shareholders on Schedule K-1-P, (3) elect and pay the optional Pass-through Entity tax for shareholders who want a federal SALT-cap workaround, and (4) reconcile any pass-through withholding required for nonresident shareholders. Skipping the form because you “had no income” is the single most expensive misconception in this area, because the late-filing penalty applies to a zero-tax return just as it applies to a six-figure return.
Before You Start: Documents and Information You Need
Pulling these documents before you open the form will save hours of back-and-forth and stop the most common rejection causes. Each item below has a direct purpose on the return.
- Federal Form 1120-S, complete with all schedules and K-1s. Illinois starts with federal ordinary income on Line 1, so a missing or unfinalized federal return makes IL-1120-ST impossible to complete accurately, and IDOR will issue a notice if your Line 1 does not match the federal return on file with the IRS.
- Federal Employer Identification Number (FEIN). The FEIN is the master key IDOR uses to match your filing to your account; a wrong digit routes payments to another taxpayer and creates a false delinquency.
- Illinois Business Tax Number (sometimes called the IL Account ID). This is your state-level identifier shown on prior IDOR correspondence and on MyTax Illinois; without it, e-filing fails at submission.
- Prior-year IL-1120-ST. You need it to carry forward Illinois net loss deductions, credit carryforwards on Schedule 1299-A, and any overpayment applied to the current year.
- Shareholder list with names, SSNs or FEINs, addresses, residency status, and ownership percentages by day. Schedule B requires every shareholder to be listed, and a missing residency code is the leading cause of pass-through withholding errors.
- Apportionment workpapers showing Illinois sales, payroll, and property if you operate in more than one state. Illinois uses single-sales-factor apportionment under 35 ILCS 5/304(h), and missing this schedule will cause IDOR to default your apportionment factor to 100%.
- Records of estimated payments and prior-year credits. These post to Step 8 and reduce what you owe; missing them causes you to overpay and wait months for a refund.
- Bank routing and account numbers for direct debit or refund deposit. Wrong digits route refunds to the wrong account, and IDOR will not chase the bank for you.
- Documentation supporting any Illinois subtractions, additions, and credits. This includes interest from U.S. obligations, river edge redevelopment zone documentation, and EDGE credit certificates from DCEO.
- A completed federal Schedule K and K-1 set. Illinois Schedule K-1-P pulls directly from the federal K-1, and a mismatch causes shareholder-level audits.
Where to Get the Form and How to Access It
The official current-year IL-1120-ST and its instruction booklet live on the IDOR business income tax forms page. Always download a fresh copy each year, because IDOR re-versions the form and its instructions even when the line numbers look unchanged.
If you plan to e-file, the easiest channel is MyTax Illinois, the state’s free taxpayer portal that supports IL-1120-ST, all required schedules, payments, and extensions. You can also e-file through any IRS-approved business tax software that participates in the IDOR MeF program, including the major commercial packages.
Paper filers can still mail the form, but Illinois pushes electronic filing for any return with more than 10 shareholders, and software-prepared returns must be e-filed unless a hardship waiver is approved under 86 Ill. Adm. Code 100.5060. Always print the form using the original PDF, not a screenshot or photocopy, because IDOR’s scanners reject forms that are not printed at 100% scale on white 8½ x 11 paper. Filers who need translated instructions can request them through IDOR’s taxpayer assistance line at 1-800-732-8866.
Step-by-Step: How to Fill Out IL-1120-ST Line by Line
The form is organized into eight numbered Steps that march from identification through the final balance due or refund. Use one tab in your workpapers per Step so nothing gets lost between drafts.
Step 1, Line A: Tax Year Beginning and Ending Dates
This box asks you to write the first and last day of the tax year you are reporting, in MM/DD/YYYY format. For a calendar-year filer, that is 01/01/2025 and 12/31/2025; for a fiscal-year filer, it is the same period covered by your federal Form 1120-S.
Enter the dates exactly as they appear on your federal return; mismatches cause IDOR’s matching system to flag the return for manual review. Maria Lopez of Lopez Design, Inc. writes 01/01/2025 and 12/31/2025. If you changed your accounting period mid-year with IRS approval, attach Form 1128 and write the short-period dates here. The most common mistake is leaving the box blank because the year is “obvious,” which causes IDOR to apply the return to the wrong year and generate a delinquency notice for the year you actually meant. A frequent misconception is that fiscal-year S corps file on the calendar deadline; they do not, because the due date keys to the close of their fiscal year.
Step 1, Line B: Federal Employer Identification Number (FEIN)
This field asks for your nine-digit federal EIN in XX-XXXXXXX format. Copy it directly from your IRS CP-575 letter or from page 1 of your federal Form 1120-S.
Enter the digits without spaces or extra dashes, and do not substitute the owner’s Social Security number. Carlos Rivera of Rivera Logistics enters 36-4881023. If you have applied for an EIN but not yet received it, you cannot file IL-1120-ST and must wait. The most common mistake is transposing two digits, which routes your filing to another taxpayer’s account and causes IDOR to bill you for a return you never filed. Many filers wrongly believe a state-issued business number can stand in for the FEIN here; only the federal EIN is accepted.
Step 1, Line C: Name and Mailing Address
This section asks for the corporation’s exact legal name, “in care of” name if applicable, street address, city, state, and ZIP+4. Write the name as it appears on the Articles of Incorporation filed with the Illinois Secretary of State.
Use all capital letters, and do not abbreviate the entity ending (write INCORPORATED if the charter says INCORPORATED). Lopez Design, Inc. writes LOPEZ DESIGN, INC., 412 N WELLS ST, CHICAGO, IL 60654-1108. If you use a P.O. Box, list it on the street line and the physical address on a separate attached statement so IDOR can reach you for audit. The most common mistake is using a DBA name instead of the legal name, which causes IDOR’s matching system to reject the return as filed by an unknown entity. A common misconception is that updating your address on this form updates it everywhere at IDOR; it does not, and you must also file Form REG-3-L.
Step 1, Line D: Illinois Business Tax (IBT) Number
This box asks for your Illinois account ID, an 8-digit number assigned when you registered with IDOR. Pull it from any IDOR letter or the top of your MyTax Illinois dashboard.
Enter the digits without dashes. Rivera Logistics enters 12345678. If you have never registered, complete Form REG-1 before you file, because returns submitted without an IBT number are returned to sender. The most common mistake is using the FEIN here by accident, which causes the return to drop into an unmatched queue. The misconception is that the IBT number and the FEIN are interchangeable; they are not.
Step 1, Lines E through M: Identifying Checkboxes
These boxes capture the entity’s status: first return, final return, name change, address change, federal audit adjustment, 52/53-week filer, automatic extension, IRS Form 8886 reportable transaction, and PTE tax election. Read every label and check only what applies.
For most active corporations, only Line M (the PTE election) gets checked. Westside Bakery, Inc., a calendar-year S corp electing PTE for 2025, checks Line M and leaves the rest blank. If you mark Final return, IDOR will close your account, so do not check that box unless the corporation has dissolved with the Secretary of State. The most common mistake is checking Automatic Extension without actually paying the tentative tax with Form IL-505-B, which leaves the extension valid for filing but invalid for payment. The misconception is that the federal extension automatically extends Illinois; the filing extension is automatic, but the payment is not.
Step 2, Line 1: Federal Ordinary Business Income or Loss
This line asks for the amount from Line 21 of your federal Form 1120-S, which is the federal ordinary business income or loss before separately stated items. Pull the number directly from the federal return, sign included.
Enter losses in parentheses (for example, (15,200)). Lopez Design, Inc. enters 84,500 because that is what appears on federal Line 21. If you amended the federal return, use the amended figure and attach a copy of the amended 1120-S. The most common mistake is mixing in separately stated items like interest or rental income, which belong on Line 2; doing so double-counts income. The misconception is that you can “smooth” income between Line 1 and Line 2; the lines are mechanical, not discretionary.
Step 2, Line 2: Net Income or Loss from All Other Activities
This line captures everything that flows through federal Schedule K but is not on Line 1, such as portfolio interest, dividends, capital gains, Section 1231 gains, and rental income. Add the federal Schedule K amounts that are not already on federal Line 21.
Show the total net of separately stated losses. Rivera Logistics adds 4,800 of interest plus 1,200 of capital gain and enters 6,000. If you have only ordinary income, enter zero rather than leaving the line blank. The most common mistake is including tax-exempt interest, which is not taxable income and inflates your base. The misconception is that this line is optional; it is not, and missing items here cause shareholder K-1-P amounts to mismatch.
Step 3, Lines 4 through 9: Additions
Step 3 lists Illinois additions to federal income, including state and municipal interest income, the Illinois income tax deducted on the federal return, and other Illinois-required addbacks. Read each line label and add only the items that apply.
Enter each addition on its specific line and total on Line 9. Westside Bakery adds back 2,400 of Illinois replacement tax it deducted federally, on Line 5. If you took bonus depreciation federally, the Illinois bonus depreciation addback goes on the Other additions line with Schedule M attached. The most common mistake is forgetting to add back the Illinois replacement tax itself, which leaves base income understated. The misconception is that interest from U.S. Treasuries gets added back; it does not, because federal obligations are protected from state tax under 31 USC 3124.
Step 4, Lines 10 through 18: Subtractions
Step 4 captures Illinois subtractions, including interest from U.S. obligations, August 1, 1969, valuation limitation amounts, river edge redevelopment income, and contributions to certain Illinois funds. Total subtractions go on Line 18.
Document each subtraction with workpapers and, where required, an attached schedule. Lopez Design subtracts 1,800 of U.S. Treasury interest on Line 12. If you claim a subtraction without supporting documentation, IDOR will deny it on audit and assess tax plus penalty. The most common mistake is double-subtracting income that was already excluded federally. The misconception is that all “interest” is subtractable; only interest on direct federal obligations qualifies.
Step 5, Line 19: Base Income or Loss
Line 19 is a math line: Line 3 plus Line 9 minus Line 18. Do the arithmetic carefully and use a calculator twice.
Enter losses in parentheses. Rivera Logistics computes 90,000 + 6,000 + 2,400 − 1,800 = 96,600. If the result is negative, that loss carries to Step 6 for apportionment and may produce an Illinois Net Loss Deduction in future years. The most common mistake is a math error that propagates through every later line. The misconception is that base income equals federal taxable income; it does not, because Illinois additions and subtractions reshape it.
Step 6, Lines 20 through 35: Apportionment for Multistate Filers
Step 6 only applies if your corporation operated in more than one state. Illinois uses a single sales factor under 35 ILCS 5/304(h), so you compute Illinois sales divided by everywhere sales and apply that percentage to your base income.
Round the factor to six decimal places, exactly as IDOR requires. Rivera Logistics has 1,400,000 of Illinois sales and 4,000,000 everywhere, producing a factor of 0.350000. If you skip Step 6 entirely and you have multistate activity, IDOR will default your apportionment to 100% Illinois. The most common mistake is sourcing services to the office that performed them; Illinois sources services to the location of the customer benefit. The misconception is that “doing business” in Illinois requires a physical office; economic nexus thresholds apply.
Step 7, Lines 36 through 49: Replacement Tax and PTE Tax
Step 7 calculates the 1.5% replacement tax on Illinois base income and, if you elected PTE on Step 1 Line M, the 4.95% PTE tax on the distributive share allocated to electing shareholders. The PTE election is irrevocable for the year and cannot be reversed if the federal benefit disappears.
Multiply Illinois base income by 0.015 for the replacement tax and by 0.0495 for PTE. Westside Bakery has 200,000 of Illinois base income, computes 3,000 of replacement tax, elects PTE, and adds 9,900 of PTE tax for a total of 12,900. If you elect PTE but have a nonresident shareholder who already had pass-through withholding under 35 ILCS 5/709.5, the withholding is not required because PTE replaces it. The most common mistake is electing PTE for a year of losses, which provides no benefit and locks in administrative complexity. The misconception is that PTE saves Illinois tax; it shifts liability from shareholders to the corporation to capture a federal SALT deduction.
Step 8, Lines 50 through 65: Payments, Credits, and Balance Due
Step 8 reconciles total tax with payments made through estimated tax (IL-1120-ST-V), prior-year overpayments applied, pass-through withholding made on your behalf, and credits from Schedule 1299-A. The balance due or refund prints on Line 65.
Enter every payment with its exact date and amount. Lopez Design paid 4 estimated payments of 750 each, totaling 3,000, applied against 2,850 of total tax for a 150 refund. If your refund is more than 1,500, IDOR may hold it for additional review under their refund integrity program. The most common mistake is forgetting to include pass-through withholding reported on a Schedule K-1-P received by your S corp from a lower-tier partnership. The misconception is that the refund automatically deposits to last year’s bank account on file; you must re-enter banking info each year.
Three Filled-Out Examples Using Real Scenarios
Scenario 1: Maria Lopez, Single-Owner Illinois-Only S Corp
Maria runs Lopez Design, Inc., a Chicago-based graphic design S corp with one shareholder (herself), all income earned in Illinois, no PTE election, and no nonresident shareholders.
| Form Section | What Maria Enters |
|---|---|
| Step 1 Line A (tax year) | 01/01/2025 to 12/31/2025 |
| Step 1 Line B (FEIN) | 36-4881023 |
| Step 1 Line C (name/address) | LOPEZ DESIGN, INC., 412 N WELLS ST, CHICAGO, IL 60654 |
| Step 1 Line D (IBT) | 12345678 |
| Step 2 Line 1 (federal ordinary income) | 84,500 |
| Step 3 Line 5 (IL replacement tax addback) | 1,268 |
| Step 4 Line 12 (U.S. interest subtraction) | 0 |
| Step 5 Line 19 (base income) | 85,768 |
| Step 7 Line 41 (replacement tax @ 1.5%) | 1,287 |
| Step 8 Line 60 (estimated payments) | 1,400 |
| Step 8 Line 65 (refund) | 113 |
Scenario 2: Carlos Rivera, Multistate S Corp Apportioning to Illinois
Carlos owns Rivera Logistics, Inc., a trucking S corp operating in Illinois, Indiana, and Wisconsin, with two resident shareholders and no PTE election.
| Form Section | What Carlos Enters |
|---|---|
| Step 1 Line A (tax year) | 01/01/2025 to 12/31/2025 |
| Step 1 Line B (FEIN) | 27-3902145 |
| Step 1 Line C (name/address) | RIVERA LOGISTICS, INC., 88 E MAIN ST, ROCKFORD, IL 61101 |
| Step 2 Line 1 (federal ordinary income) | 240,000 |
| Step 2 Line 2 (other federal Sch K items) | 6,000 |
| Step 5 Line 19 (base income) | 252,400 |
| Step 6 Line 32 (apportionment factor) | 0.350000 |
| Step 6 Line 35 (Illinois base income) | 88,340 |
| Step 7 Line 41 (replacement tax) | 1,325 |
| Step 8 Line 60 (estimated payments) | 1,000 |
| Step 8 Line 65 (balance due) | 325 |
Scenario 3: Aisha Khan, S Corp Electing PTE Tax for Nonresident Shareholders
Aisha is the managing shareholder of Westside Bakery, Inc., a 4-shareholder Illinois S corp with two nonresident shareholders in Wisconsin, electing PTE to give those shareholders a federal SALT deduction.
| Form Section | What Aisha Enters |
|---|---|
| Step 1 Line A (tax year) | 01/01/2025 to 12/31/2025 |
| Step 1 Line B (FEIN) | 46-7710288 |
| Step 1 Line M (PTE election checkbox) | Checked |
| Step 2 Line 1 (federal ordinary income) | 198,000 |
| Step 5 Line 19 (base income) | 200,000 |
| Step 7 Line 41 (replacement tax) | 3,000 |
| Step 7 Line 47 (PTE tax @ 4.95%) | 9,900 |
| Step 7 Line 49 (total tax) | 12,900 |
| Step 8 Line 60 (estimated payments) | 13,000 |
| Step 8 Line 65 (refund) | 100 |
| Schedule B (shareholder list) | 4 shareholders listed with residency codes |
How to File the Completed Form
Online via MyTax Illinois. This is the preferred channel for almost every filer. Log in with your IDOR credentials, choose File a Return, select IL-1120-ST, upload required schedules, and pay any balance via direct debit (ACH) or credit card (a 2.25% convenience fee applies through the third-party processor). Processing is same-day for the filing acknowledgment, refunds typically post within 4 weeks, and your proof of filing is the confirmation number plus a downloadable PDF receipt.
Through approved tax software. Any IRS Modernized e-File (MeF) participant that supports Illinois business returns, including the major commercial software brands, can transmit IL-1120-ST and all schedules in one package. Fees vary by vendor. Processing is identical to MyTax Illinois, and proof of filing is the software’s electronic acknowledgment, which you should download and store with the return.
By paper. Mail returns with a payment to Illinois Department of Revenue, P.O. Box 19053, Springfield, IL 62794-9053; mail returns expecting a refund or with no payment to Illinois Department of Revenue, P.O. Box 19032, Springfield, IL 62794-9032. Use Form IL-1120-ST-V as your payment voucher if paying by check. Processing time is 8 to 12 weeks, sometimes longer at peak season, and proof of filing is your USPS Certified Mail receipt with green card; never mail without tracking.
By in-person drop-off. IDOR offices in Springfield, Chicago, Des Plaines, Rockford, and Marion accept hand-delivered returns during business hours; bring two copies and ask the clerk to date-stamp your file copy. There is no separate fee, payment is by check or money order, and your stamped copy is the proof of filing.
By fax. IDOR does not accept IL-1120-ST by fax under any circumstance, and faxed returns are treated as never filed.
What Happens After You File
IDOR’s matching system checks your return against your federal Form 1120-S, prior-year IDOR records, and Schedule K-1-P data reported by other pass-through entities. If everything matches, the return moves to accepted status within 24 hours of e-filing, and any refund is issued within 4 weeks via the deposit method you selected.
If the matching system flags a discrepancy, you receive an IDOR notice such as a Return Correction Notice (form IDR-393-RC) or a Notice of Deficiency, both of which give you 60 days to respond before the assessment becomes final. Ignoring a notice is the single fastest way to convert a small math error into a lien on the corporation’s bank account.
If you owe additional tax after filing, interest under 35 ILCS 735/3-2 accrues daily from the original due date until paid. If the IRS later adjusts your federal Form 1120-S, you must file an amended IL-1120-ST within 120 days of the federal change under 35 ILCS 5/506(b); missing that 120-day window converts a routine adjustment into a penalty event.
Mistakes to Avoid When Filling Out the Form
- Filing IL-1120 instead of IL-1120-ST. This treats the S corp as a C corp; IDOR will reject the return and may assess penalties on a missing IL-1120-ST.
- Forgetting to add back Illinois replacement tax on Step 3. The omission understates base income and triggers an automatic IDOR adjustment plus interest.
- Skipping Schedule B. Without the shareholder list, IDOR cannot match Schedule K-1-P amounts, and the return is treated as incomplete.
- Defaulting apportionment to 100% by skipping Step 6. Multistate filers who skip apportionment overpay Illinois tax and lose the right to refund after the statute closes.
- Electing PTE in a loss year. The election is irrevocable for the year, locks in compliance complexity, and provides no federal benefit when there is no Illinois income.
- Mismatching Line 1 with federal Form 1120-S Line 21. Any difference triggers the IDOR matching algorithm and stops processing.
- Using the FEIN where the IBT number is required, or vice versa. This routes the return to an unmatched queue and creates a phantom delinquency.
- Forgetting pass-through withholding credits on Step 8. Filers who received Schedule K-1-P from a partnership lose the credit and overpay.
- Filing on paper when e-file is required. Software-prepared returns must be e-filed, and paper submissions can be rejected outright.
- Missing the 120-day federal change reporting deadline. This converts a routine federal RAR into a state penalty case.
- Filing without paying the tentative tax with IL-505-B when extending. The filing extension is automatic, but the payment is not, and unpaid tax accrues penalty and interest.
Do’s and Don’ts
- Do download the current-year form from IDOR every year, because line numbers and schedules change.
- Do reconcile every line to your federal Form 1120-S before filing, because IDOR’s matching system is unforgiving.
- Do e-file through MyTax Illinois whenever possible to get same-day proof of filing.
- Do keep workpapers for at least seven years, because IDOR can reach back further than the IRS in some scenarios.
- Do pay any balance due electronically by the original due date even if you extend, because the extension does not extend payment.
- Do model the PTE election before you check Line M, because it is irrevocable for the year.
- Don’t check Final Return unless the corporation has actually dissolved with the Secretary of State.
- Don’t abbreviate the entity name or use a DBA where the legal name is required.
- Don’t assume the federal extension extends Illinois payment; it does not.
- Don’t mail without certified tracking, because lost paper returns are deemed never filed.
- Don’t ignore an IDOR notice; the 60-day response window controls your appeal rights.
- Don’t mix tax years in one envelope, because each year must be in its own filing.
Pros and Cons of Filing on Your Own vs. With Help
| Approach | Why it Works or Doesn’t |
|---|---|
| DIY with MyTax Illinois (Pro) | Free, fast, same-day acknowledgment, and built-in math checks reduce errors. |
| DIY with software (Pro) | Imports federal data automatically and supports all Illinois schedules. |
| DIY paper (Pro) | Works for very small, single-shareholder, Illinois-only S corps with no schedules. |
| DIY (Con) | Easy to miss the PTE election analysis, apportionment rules, and credit carryforwards. |
| DIY (Con) | No professional liability backstop if IDOR audits and assesses tax. |
| CPA preparation (Pro) | Catches PTE planning, apportionment nuance, and credit opportunities like EDGE and 1299-A. |
| CPA preparation (Pro) | Provides representation if IDOR issues a notice or audit. |
| CPA preparation (Pro) | Aligns Illinois return with federal 1120-S and shareholder 1040s. |
| CPA preparation (Con) | Fees typically run $750 to $3,000 for an IL-1120-ST package. |
| CPA preparation (Con) | Requires earlier delivery of records to meet the March 15 deadline. |
Comparing IL-1120-ST to Related Forms
| Form | Purpose |
|---|---|
| IL-1120-ST | Annual return for S corporations to report income and pay 1.5% replacement tax. |
| IL-1120 | Annual return for C corporations, paying 7% income tax plus 2.5% replacement tax. |
| IL-1065 | Annual return for partnerships, also paying 1.5% replacement tax. |
| IL-1040 | Annual return for individual shareholders to report their distributive share. |
| IL-505-B | Extension payment voucher for business income tax. |
| Schedule K-1-P | Pass-through statement issued to each shareholder. |
| Schedule B | Shareholder list filed with IL-1120-ST. |
FAQs
Who must file Illinois Form IL-1120-ST?
Yes. Every S corporation organized in, qualified in, or earning income from Illinois must file IL-1120-ST, even with zero Illinois income, under 35 ILCS 5/502.
When is IL-1120-ST due?
Yes. It is due the 15th day of the 3rd month after year-end, which is March 15 for calendar-year filers, with an automatic 7-month extension to file but not to pay.
Is the 7-month extension automatic?
Yes. Illinois grants an automatic 7-month filing extension without a request, but you must still pay the tentative tax with Form IL-505-B by the original deadline to avoid penalty.
Is the PTE tax election irrevocable?
Yes. Once you check Step 1 Line M for the year, the election cannot be revoked, even if the federal SALT-cap benefit disappears later.
Do I write my legal name or DBA on Step 1 Line C?
No. Use the legal name from your Articles of Incorporation; using a DBA causes the IDOR matching system to reject the return as filed by an unknown entity.
Do I list every shareholder on Schedule B?
Yes. You must list every shareholder who held stock during the year with name, ID, address, residency, and ownership percentage, regardless of how briefly they held shares.
Can I e-file IL-1120-ST?
Yes. Use MyTax Illinois or any IRS Modernized e-File participant that supports Illinois business returns; e-file is required for most software-prepared returns.
Is replacement tax deductible federally?
Yes. It is deductible on the federal return as a state tax, but you must add it back on Step 3 Line 5 of the Illinois return.
Do I use the FEIN or the IBT number on Step 1 Line D?
No. Line D requires the 8-digit Illinois Business Tax (IBT) number, not the federal EIN; substituting the FEIN drops the return into IDOR’s unmatched queue.
Are U.S. Treasury interest amounts subtractable?
Yes. Interest from direct U.S. obligations is subtractable on Step 4 Line 12 under federal preemption in 31 USC 3124.
Can I file IL-1120-ST by fax?
No. IDOR does not accept any IL-1120-ST submission by fax, and faxed returns are treated as never filed.
What if the IRS later changes my federal return?
Yes. You must file an amended IL-1120-ST within 120 days of the federal change under 35 ILCS 5/506(b); missing that window triggers penalty.
Does electing PTE eliminate pass-through withholding for nonresidents?
Yes. The PTE election under 35 ILCS 5/201(p) replaces the pass-through withholding obligation for the electing shareholders’ Illinois share of income.
Do I need to attach federal Form 1120-S?
Yes. A complete copy of the federal Form 1120-S, including Schedules K and K-1, must accompany IL-1120-ST, and missing federal attachments cause IDOR to suspend processing.
Related reading
- How to Fill Out Illinois Form IL-1041 (w/Examples) + FAQs
- How to Fill Out Illinois Form IL-1120-V (w/Examples) + FAQs
- How to Fill Out Illinois Form IL-1120 (w/Examples) + FAQs
- How to Fill Out Illinois Form IL-1120-X (w/Examples) + FAQs
- How to Fill Out Illinois Form IL-941 (w/Examples) + FAQs
- How to Fill Out Illinois Form IL-941-X (w/Examples) + FAQs