As the owner of the vehicle, the lessor is liable for Illinois Use Tax and responsible for filing and paying tax using Form RUT-25 when the vehicle is brought into Illinois. However, you, the lessee, may assume this responsibility. If you do so, you should attach a copy of the bill of sale or other proof of the purchase price. Other proof may be in the form of a letter from the leasing company or selling dealer, a fair market valuation from a licensed dealer, or a copy of the lease agreement.