820 ILCS 405/1401
Any employer who shall fail to pay any contributions (including any amounts due pursuant to Section 1506.3) when required of him by the provisions of this Act and the rules and regulations of the Director, whether or not the amount thereof has been determined and assessed by the Director, shall pay to the Department , in addition to such contribution, interest thereon at the rate of one percent (1%) per month and one-thirtieth (1/30) of one percent (1%) for each day or fraction thereof computed from the day upon which said contribution became due. After 1981, such interest shall accrue at the rate of 2% per month, computed at the rate of 12/365 of 2% for each day or fraction thereof, upon any unpaid contributions which become due, provided that, after 1987, for the purposes of calculating interest due under this Section only, payments received more than 30 days after such contributions become due shall be deemed received on the last day of the month preceding the month in which they were received except that, if the last day of such preceding month is less than 30 days after the date that such contributions became due, then such payments shall be deemed to have been received on the 30th day after the date such contributions became due.
However, all or part of any interest may be waived by the Director for good cause shown.
820 ILCS 405/1401