Ill COMP STAT § 10 : Illinois Statutes - Section 10

820 ILCS 112/10)
Sec. 10. Prohibited acts.
(a) No employer may discriminate between employees on the basis of sex by paying wages to an employee at a rate less than the rate at which the employer pays wages to another employee of the opposite sex for the same or substantially similar work on jobs the performance of which requires equal skill, effort, and responsibility, and which are performed under similar working conditions, except where the payment is made under:
(1) a seniority system;
(2) a merit system;
(3) a system that measures earnings by quantity or quality of production; or
(4) a differential based on any other factor other than: (i) sex or (ii) a factor that would constitute unlawful discrimination under the Illinois Human Rights Act.
An employer who is paying wages in violation of this Act may not, to comply with this Act, reduce the wages of any other employee.
Nothing in this Act may be construed to require an employer to pay, to any employee at a workplace in a particular county, wages that are equal to the wages paid by that employer at a workplace in another county to employees in jobs the performance of which requires equal skill, effort, and responsibility, and which are performed under similar working conditions.
(b) It is unlawful for any employer to interfere with, restrain, or deny the exercise of or the attempt to exercise any right provided under this Act. It is unlawful for any employer to discharge or in any other manner discriminate against any individual for inquiring about, disclosing, comparing, or otherwise discussing the employee's wages or the wages of any other employee, or aiding or encouraging any person to exercise his or her rights under this Act.
(c) It is unlawful for any person to discharge or in any other manner discriminate against any individual because the individual:
(1) has filed any charge or has instituted or caused to be instituted any proceeding under or related to this Act;
(2) has given, or is about to give, any information in connection with any inquiry or proceeding relating to any right provided under this Act; or
(3) has testified, or is about to testify, in any inquiry or proceeding relating to any right provided under this Act.
(Source: P.A. 93‑6, eff. 1‑1‑04.)