You report it in both states. IL can tax a capital gain from the sale of property located in IL. CO can tax ALL your income, regardless of where earned.The Tax Department is streamlining withholding tax and wage reporting forms and Web File applications. You can deduct real estate taxes imposed on you. Iowa and Illinois have a reciprocal agreement for individual income tax purposes. At this time, Iowa's only income tax reciprocal agreement is with Illinois. Illinois has a flat income tax rate of 4.95 percent. Illinois has a 9.5 percent corporate income tax rate. No information is available for this page. The state's sales tax is imposed on the purchase price of tangible personal property or taxable service sold at retail.