When you file a federal return with Social Security benefits, there is a calculation on the federal return to determine if any of the Social Security benefits are taxable, and if so, how much. Up to 85% of SS income may be taxable.

When you come to your Illinois return, the amount of SS income that is taxable should be added to line 5 on the IL-1040. This amount is eventually subtracted from federal income to create IL taxable income (line 11).

What do you see that tells you that Illinois is taxing your SS income?