Illinois Quietly Rewrote Its Business Tax Rules
While national attention has focused on states cutting income taxes, Illinois moved in a different direction. As part of its 2026 budget legislation, the state enacted a series of business tax changes that will significantly increase the tax burden for multi-state and multinational companies operating in Illinois.
These aren't headline-grabbing rate increases. They're structural changes to how Illinois calculates taxable income for businesses — the kind of technical shifts that don't trend on social media but can mean millions of dollars in additional tax liability for affected companies.
The Big One: Joyce to Finnigan
The most consequential change is Illinois's shift from the Joyce method to the Finnigan method of sales apportionment for combined (unitary) corporate groups, effective for tax years ending on or after December 31, 2025.
What This Means in Plain English
When a corporation operates in multiple states, each state gets to tax a portion of that company's income. The most common way to determine each state's share is sales apportionment — the percentage of the company's total sales that occur in a given state determines how much income that state can tax.
The complication arises when you have a unitary group: a parent company and its subsidiaries that operate as a single economic unit. Illinois requires these groups to file combined returns. The question is: whose sales count when determining the Illinois apportionment percentage?
Under Joyce (the old rule):
Only the sales of group members that individually have nexus (a taxable connection) with Illinois are included in the apportionment calculation. If a subsidiary has no employees, property, or significant sales in Illinois, its sales are excluded from the numerator — even if it's part of a combined filing group.
Under Finnigan (the new rule):
All unitary group members' sales in Illinois count toward the apportionment numerator — even if the individual entity making those sales doesn't independently have nexus with Illinois.
Why This Matters
Under Joyce, sophisticated corporate groups could structure their operations so that entities with the most Illinois sales lacked nexus, while the entity with nexus had relatively few Illinois sales. The result: a lower apportionment percentage and less income taxed by Illinois.
Finnigan closes that door. If any member of your unitary group has nexus with Illinois, all Illinois sales by any group member get pulled into the calculation.
Example
Consider a unitary group with three entities:
| Entity | IL Nexus? | IL Sales | Total Sales |
|---|
| Parent Corp | Yes | $5M | $100M |
| Sub A | No | $20M | $80M |
| Sub B | No | $15M | $60M |
| Combined | — | $40M | $240M |
Under Joyce: Only Parent Corp's IL sales count in the numerator. Apportionment = $5M / $240M = 2.08%
Under Finnigan: All IL sales count. Apportionment = $40M / $240M = 16.67%
On $50 million in combined group income, that's the difference between Illinois taxing $1.04 million and $8.33 million of income. At Illinois's combined corporate income and replacement tax rate of approximately 9.5%, the additional tax is roughly $693,000 in this simplified example.
For large corporate groups with significant Illinois sales spread across multiple entities, the impact can run into the tens of millions.
GILTI Deduction Cut in Half
Illinois also reduced the deduction for Global Intangible Low-Taxed Income (GILTI) from 100% to 50%, for tax years ending on or after December 31, 2025.
Background
GILTI is a category of foreign income created by the 2017 federal tax reform. It's designed to prevent companies from parking profits in low-tax foreign jurisdictions. At the federal level, companies can deduct 50% of GILTI (effectively taxing it at a reduced rate).
Until this change, Illinois allowed a full 100% deduction for GILTI — meaning the state effectively didn't tax this income at all. The new 50% limitation aligns Illinois more closely with the federal treatment and brings foreign income into the Illinois tax base.
Impact
Companies with substantial foreign operations — particularly technology, pharmaceutical, and manufacturing firms — will see their Illinois taxable income increase by the amount of GILTI that's no longer deductible. For a company with $100 million in GILTI, the additional Illinois tax is approximately $4.75 million annually.
Closing the Transfer Pricing Loopholes
Illinois has long required corporations to "add back" certain intercompany interest and intangible expenses to their taxable income. This prevents companies from using related-party payments to shift income out of Illinois.
Previously, there were exceptions to these addback rules — for instance, if the recipient entity was subject to tax in another jurisdiction or if the transaction had a legitimate business purpose. The 2026 changes eliminate the non-tax-avoidance and subject-to-tax exceptions, making the addback rules more broadly applicable.
In practical terms, this means:
- Interest payments between related entities are more likely to be added back to Illinois income, even if the recipient pays tax elsewhere
- Royalty and licensing payments for intangible property face tighter scrutiny
- Companies relying on intercompany financing structures to reduce Illinois taxable income need to reassess those arrangements
Remote Seller Nexus: Simplified
Illinois previously used a two-prong nexus test for remote sellers: a business had nexus if it either (a) exceeded $100,000 in sales to Illinois customers or (b) completed 200 or more transactions with Illinois customers in a year.
The 200-transaction threshold has been eliminated. Nexus is now determined solely by the revenue threshold.
This is actually a simplification that benefits small businesses. A company selling inexpensive items could previously trigger nexus through volume alone — 200 transactions at $5 each is only $1,000 in sales but was enough to create a filing obligation. Now, only the dollar amount matters.
New Amnesty Programs
As part of the broader package, Illinois is offering tax amnesty programs that allow businesses to settle outstanding tax liabilities at reduced penalties and interest. The specific terms and timelines are administered by the Illinois Department of Revenue.
Amnesty programs typically benefit companies that have underpaid or failed to file in prior years — often because they didn't realize they had nexus with Illinois. Given the expanded reach of the Finnigan rule, this amnesty is well-timed: companies that now clearly have Illinois exposure can come into compliance with reduced risk.
Consumer-Facing Tax Increases
While the structural changes above are business-focused, Illinois also enacted several rate increases on consumer-facing taxes:
- Tobacco taxes: Increased on cigarettes and other tobacco products
- Sports betting: Wagering tax rates increased on licensed operators
- Telecom taxes: Increased surcharges on telecommunications services
These don't change the personal income tax rate (which remains at 4.95% flat) but add to the overall cost of living and doing business in Illinois.
The Broader Context
Illinois's changes run counter to the national trend of states cutting taxes to attract businesses and residents. While 26 states have reduced income tax rates since 2021, Illinois has effectively increased its corporate tax reach without touching the headline rate.
This matters for the state's competitive position:
- Indiana (next door) has been steadily cutting its corporate rate and now sits at 4.9%, roughly half of Illinois's combined rate
- Iowa flattened and reduced its rates dramatically
- Missouri and Kentucky are both on downward trajectories
Illinois's defenders argue that the state's economy — anchored by Chicago's financial sector, major universities, and transportation infrastructure — can absorb these changes without driving away business. Critics see it as another reason for the outmigration trend that has cost Illinois a congressional seat in each of the last two reapportionments.
What Businesses Should Do Now
If your business operates in Illinois as part of a multi-state or multinational group, these changes demand attention:
- Remodel your apportionment under Finnigan rules to understand the actual tax impact on your combined group
- Reassess intercompany transactions in light of the tighter addback requirements
- Quantify your GILTI exposure at the state level now that the deduction has been halved
- Consider the amnesty program if you have potential prior-year exposure
- Evaluate your entity structure — the Finnigan shift may change the calculus on which entities should or shouldn't be part of your Illinois combined return
These are complex, fact-specific analyses. The dollar amounts at stake — particularly for the Joyce-to-Finnigan shift — are large enough to justify immediate review with a state tax specialist.
This article is for informational purposes only and does not constitute tax advice. Consult a qualified tax professional for guidance specific to your situation.