35 ILCS 615/2
A tax is imposed upon persons engaged in the business of distributing, supplying, furnishing or selling gas to persons for use or consumption and not for resale at the rate of 2.4 cents per therm of all gas which is so distributed, supplied, furnished, sold or transported to or for each customer in the course of such business, or 5% of the gross receipts received from each customer from such business, whichever is the lower rate as applied to each customer for that customer's billing period, provided that any change in rate imposed by this amendatory Act of 1985 shall become effective only with bills having a meter reading date on or after January 1, 1986. However, such taxes are not imposed with respect to any business in interstate commerce, or otherwise to the extent to which such business may not, under the Constitution and statutes of the United States, be made the subject of taxation by this State.
Nothing in this amendatory Act of 1985 shall impose a tax with respect to any transaction with respect to which no tax was imposed immediately preceding the effective date of this amendatory Act of 1985.
Beginning with bills issued to customers on and after October 1, 2003, no tax shall be imposed under this Act on transactions with customers who incur a tax liability under the Gas Use Tax Law.
35 ILCS 615/2