35 ILCS 120/5i
Any person who incurs tax liability under this Act, and who removes from this State or conceals his whereabouts, shall be deemed thereby to appoint the Secretary of State of Illinois his agent for service of process or notice in any judicial or administrative proceeding under this Act. Such process or notice shall be served by the Department on the Secretary of State by leaving, at the office of the Secretary of State at least 15 days before the return day of such process or notice, a true and certified copy thereof, and by sending to the taxpayer by registered or certified mail, postage prepaid, a like and true certified copy, with an endorsement thereon of the service upon said Secretary of State, addressed to such taxpayer at his last known address.
Service of process or notice in the manner provided for in this Section, under the circumstances specified in this Section, shall be of the same force and validity as if served upon the taxpayer personally within this State. Proof of such service upon the taxpayer in this State through the Secretary of State as his agent and by mailing to the last known address of the taxpayer may be made in such judicial or administrative proceeding by the affidavit of the Director of Revenue, or by his duly authorized representative who made such service, with a copy of the process or notice that was so served attached to such affidavit.
35 ILCS 120/5i