1. a) Where 2 or more persons are engaged in a unitary business as described in IITA Section 1501(a)(27), a part of which is conducted in this State by one or more members of the group, the business income attributable to this State by any such member or members shall be apportioned by means of the combined apportionment method. (IITA Section 304(e))
2. b) All members of a unitary business group must use the combined apportionment method to determine business income attributable to Illinois, including the provisions for determining taxability in another state as set forth in Section 100.3200 of this Part.
3. c) The combined apportionment method is applied by first computing the business income of each member of the unitary business group to derive the total business income of the group. Next, the apportionment factor for each group member subject to Illinois income tax is computed using the individual group member's Illinois sales as the numerator and the entire unitary business group's sales as the denominator. This apportionment factor is applied to the group's total business income to derive the amount of business income on which the group member would pay Illinois income tax. (See General Telephone Co. v. Johnson, 469 N.E.2d 1067 (Ill. 1984).)
4. d) For tax years ending on or after December 31, 2025, sales of each member of a unitary business group who is not a taxpayer, as defined in IITA Section 1501(a)(24), shall be determined based upon the apportionment rules applicable to the member and shall be aggregated. Each taxpayer member of the unitary business group shall include in its sales factor numerator a portion of the aggregate Illinois sales of the non-taxpayer members based on a ratio, the numerator of which is that taxpayer member's Illinois sales taking into account its applicable sales factor provisions, and the denominator of which is the aggregate Illinois sales of all the taxpayer members of the group taking into account their respective sales factor provisions. In addition, if inclusion of sales in the sales factor or numerator of the sales factor depends on whether a taxpayer is considered taxable in another state within the meaning of IITA Section 303(f), that taxpayer shall be considered taxable in any state in which any member of its unitary business group is considered taxable under IITA Section 303(f). (IITA Section 304(e))

5. e) The following examples illustrate the provisions of this Section:

   EXAMPLE 1: Corporations A, B, and C constitute a unitary business group. All members have a taxable year ending June 30, 2024, and all members are taxable in Illinois. Corporation A has $5,000,000 in business income, $1,000,000 in Illinois sales, and $5,000,000 in everywhere sales. Corporation B has $2,000,000 in business income, $1,500,000 in Illinois sales, and $2,000,000 in everywhere sales. Corporation C has $3,000,000 in business income, $2,000,000 in Illinois sales, and $3,000,000 in everywhere sales. Total combined apportionable income is $10,000,000. The combined income apportionable to Illinois for the common tax year is computed as follows: $10,000,000 in combined business income x ($4,500,000 of A, B, and C's Illinois sales/$10,000,000 of combined total sales) = $4,500,000.

   EXAMPLE 2: Corporations X, Y, and Z constitute a unitary business group. All members have a taxable year ending June 30, 2024. Corporation Z is protected by Public Law 86-272 and not taxable in Illinois. Corporation X has $800,000 in business income, $600,000 in Illinois sales, and $800,000 in everywhere sales. Corporation Y has $1,000,000 in business income, $500,000 in Illinois sales, and $1,000,000 in everywhere sales. Corporation Z has $4,000,000 in business income, $200,000 in Illinois sales, and $4,000,000 in everywhere sales. Total combined apportionable income is $5,800,000. The combined income apportionable to Illinois for the common tax year is computed as follows: $5,800,000 in combined business income x ($1,100,000 of X and Y's Illinois sales/$5,800,000 of combined total sales) = $1,100,000.

   EXAMPLE 3: Corporations D, E, and F constitute a unitary business group. All members have a taxable year ending December 31, 2025. Corporation F is protected by Public Law 86-272 and not taxable in Illinois. Corporation D has $800,000 in business income, $600,000 in Illinois sales, and $800,000 in everywhere sales. Corporation E has $1,000,000 in business income, $500,000 in Illinois sales, and $1,000,000 in everywhere sales. Corporation F has $4,000,000 in business income, $200,000 in Illinois sales, and $4,000,000 in everywhere sales. Total combined apportionable income is $5,800,000. Corporation D must include in its sales factor numerator $109,091 of Corporation F's Illinois sales computed as follows: $200,000 of F's Illinois sales x ($600,000 of D's Illinois sales/$1,100,000 of D and E's combined Illinois sales). Corporation E must include in its sales factor numerator $90,909 of Corporation F's Illinois sales computed as follows: $200,000 of F's Illinois sales x ($500,000 of E's Illinois sales/$1,100,000 of D and E's combined Illinois sales). The combined income apportionable to Illinois for the common tax year is computed as follows: $5,800,000 in combined business income x [($600,000 D's Illinois sales + $109,091 F's apportioned Illinois sales)/$5,800,000 of combined total sales + ($500,000 E's Illinois sales + $90,909 F's apportioned Illinois sales)/$5,800,000 of combined total sales] = $1,300,000.

   EXAMPLE 4: Corporations R, S, and T constitute a unitary business group. All members have a taxable year ending December 31, 2025. Corporation T is protected by Public Law 86-272 and not taxable in Illinois. Corporation T has $500,000 in sales from Illinois to customers in State M, where one or more members of the unitary business group has taxable nexus. As at least one member of the unitary business group has taxable nexus in State M, Illinois' throwback rule would not apply to the sales made by Corporation T to customers in State M. These sales are considered taxable in another state because the unitary business group has a connection to State M. The combined sales factor denominator remains the total combined sales of the group.

(Source: Added at 50 Ill. Reg. 8595, effective June 2, 2026)