305 ILCS 5/10-16.5
A support obligation, or any portion of a support obligation, which becomes due and remains unpaid as of the end of each month, excluding the child support that was due for that month to the extent that it was not paid in that month, shall accrue simple interest as set forth in Section 12-109 of the Code of Civil Procedure. An order for support entered or modified on or after January 1, 2006 shall contain a statement that a support obligation required under the order, or any portion of a support obligation required under the order, that becomes due and remains unpaid as of the end of each month, excluding the child support that was due for that month to the extent that it was not paid in that month, shall accrue simple interest as set forth in Section 12-109 of the Code of Civil Procedure. Failure to include the statement in the order for support does not affect the validity of the order or the accrual of interest as provided in this Section. The Department may provide, by rule, if, or how, the Department will enforce interest in cases in which IV-D services are being provided.
305 ILCS 5/10-16.5