I am an out-of-state employer that voluntarily withheld Illinois taxes as a courtesy for my employee, how do I report this on their W-2?

All of the wages will be listed in the state in which the person worked. You will only list Illinois wages if they worked in Illinois, and then, only for the work performed in Illinois. If they did not work in Illinois, you will list zero (0) wages in Box 16 and the amount of Illinois withholding in Box 17 on Form W-2.

If you have additional inquiries, you may submit them to the Questions, Comments, or Request form.