If the out-of-state leasing company is registered with the Illinois Department of Revenue to collect Illinois tax, then Form ST-556 is used to report the purchase. Generally, the out-of-state leasing company will complete and file this tax return along with the required title forms for you.
If the out-of-state leasing company is not registered, you will be required to report your purchase. To do so, you must use Form RUT-25. Since Form RUT-25 is due no later than 30 days after the date the vehicle is brought into Illinois and you already have possession of the vehicle in Illinois, use the actual purchase date for both the "purchase date" and the "date brought into Illinois" on the return. You should attach a copy of the bill of sale as proof of purchase price.