Illinois has a flat tax of 3.75%
Social Security benefits are not taxed by the state of Illinois.
If you receive a distribution from a qualified retirement plan, such as a 401(k), IRA, self-employment retirement, traditional IRAs that have been moved to Roth IRAs, U.S. Retirement bonds that have been redeemed, deferred pensions from state and local government service, disability and retirement plans from the government and military plans you are exempt from state taxes.
Any Illinois residents who earned income in Iowa, Kentucky, Michigan or Wisconsin are not taxed by the state of Illinois. Based on a reciprocal agreement with those state they should not levy state taxes for residents of Illinois.