Illinois (IL) Corporate Tax Rate, Economic Nexus, Apportionment & IRC §174 Conformity Guide (2025/2026)
Illinois corporate tax guide for CFOs and CPAs: 9.50% Corp Income + Replacement (9.5%), 100% Single Sales apportionment, $100,000 economic nexus threshold, IRC §174 R&D conformity, and §168(k) bonus depreciation rules.
1. How Illinois Apportionment & Economic Nexus Impact ASC 740 Tax Provisions
For multi-state C-Corporations and SaaS companies selling into Illinois (IL), exceeding $100,000 in annual IL-sourced receipts (or maintaining remote W-2 employees inside the state) establishes nexus under Illinois's Corp Income + Replacement (9.5%). Under 100% Single Sales apportionment and Market-Based rules, SaaS subscription revenue is sourced to Illinois based on where the customer receives the benefit of the software.
2. Illinois Federal-to-State Tax Base Modifications (§174 R&D & §168(k) Bonus Depreciation)
- IRC §174 R&D Capitalization Rule in Illinois: Illinois conforms to the federal TCJA mandatory 5-year amortization of domestic research and experimental (R&E) expenditures under IRC §174. No state subtraction modification is permitted.
- IRC §168(k) Bonus Depreciation Rule in Illinois: Illinois decouples from federal IRC §168(k) bonus depreciation. Corporate taxpayers must add back federal bonus depreciation on the IL return and track a separate state depreciation basis rollforward.
- Pass-Through Entity Tax (PTET) Election: Illinois offers an elective Pass-Through Entity Tax (PTET) at an effective rate of 4.95% for S-Corporations and Partnerships (1065/1120-S) looking to bypass the $10,000 federal SALT deduction cap.
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