35 ILCS 173/5-10
Beginning October 1, 2003, a tax is imposed upon the privilege of using in this State gas obtained in a purchase of out-of-state gas at the rate of 2.4 cents per therm or 5% of the purchase price for the billing period, whichever is the lower rate. Such tax rate shall be referred to as the "self-assessing purchaser tax rate". Beginning with bills issued by delivering suppliers on and after October 1, 2003, purchasers may elect an alternative tax rate of 2.4 cents per therm to be paid under the provisions of Section 5-15 of this Law to a delivering supplier maintaining a place of business in this State. Such tax rate shall be referred to as the "alternate tax rate". The tax imposed under this Section shall not apply to gas used by business enterprises certified under Section 9-222.1 of the Public Utilities Act or Section 605-1115 of the Department of Commerce and Economic Opportunity Law of the Civil Administrative Code of Illinois, as amended, to the extent of such exemption and during the period of time specified by the Department of Commerce and Economic Opportunity.
35 ILCS 173/5-10