This article explains how spousal maintenance payments impact child support obligations in Illinois.
Under Illinois, child support law, which you can read all about here, each parent's responsibility for child support is determined based on their "net incomes" relative to one another. According to 750 ILCS 5/505(a)(2)(A), a portion of the Illinois Marriage and Dissolution of Marriage Act, maintenance payments received by one of the parents must be included in "gross income" for the purpose of calculating child support. Since each parent's child support obligation is based on the parents' relative income, the higher your income compared to the other parent, the less you will receive in child support.
Therefore, if you are the recipient of child support, any maintenance payments that you receive, whether from the payor parent or a different ex-spouse, are likely to reduce the payor parent's child support obligation.
The flip-side of that coin is that 750 ILCS 5/505(a)(2)(B) provides that the "net income" of the payor parent is reduced by any maintenance obligations paid to the "same party to whom child support is payable." Maintenance payments to third parties will not reduce "net income." Still, maintenance payments to the other parent in a child support action will reduce the payor parent's net income accordingly and, therefore, will likely reduce the payor's ultimate child support obligation.
So, the bottom line is that maintenance payments paid from one parent to the other effect both parents' "net incomes" to calculate each parent's child support obligation, likely reducing the child support obligation of the party making maintenance payments. Maintenance payments from a party other than the other parent will increase your "net income," thereby likely increasing the portion of child support for which you are responsible. However, maintenance payments made to a third party other than the other parent will not reduce net income and do not affect your child support obligation.