35 ILCS 615/2a.1
In addition to the taxes imposed by the Illinois Income Tax Act and Section 2 of this Act, there is hereby imposed upon persons engaged in the business of distributing, supplying, furnishing or selling gas and subject to the tax imposed by this Act (other than a school district or unit of local government as defined in Section 1 of Article VII of the Illinois Constitution of 1970), an additional tax in an amount equal to .8% of such persons' invested capital for the taxable period. If such persons are not liable for such additional tax for the entire taxable period, such additional tax shall be computed on the portion of the taxable period during which such persons were liable for such additional tax. The invested capital tax imposed by this Section shall not be imposed upon persons who are not regulated by the Illinois Commerce Commission. Provided, in the case of any person which is subject to the invested capital tax imposed by this Section and which is also subject to the tax on the distribution of electricity imposed by Section 2a.1 of the Public Utilities Revenue Act, for taxable periods beginning on or after January 1, 1998, the invested capital tax imposed by this Section shall be the lesser of (i) an amount equal to 0.8% of such person's invested capital for the taxable period multiplied by a fraction the numerator of which is the average of the beginning and ending balances of such person's gross gas utility plant in service and the denominator of which is the average of the beginning and ending balances of such person's gross electric and gas utility plant in service, as set forth in such person's annual report to the Illinois Commerce Commission for the taxable period, or (ii) an amount equal to 0.8% of the person's invested capital for the taxable period ended December 31, 1996 multiplied by a fraction the numerator of which is the average of the beginning and ending balances of the person's gross gas utility plant in service and the denominator of which is the average of the beginning and ending balances of the person's gross electric and gas utility plant in service as set forth in the person's annual report to the Illinois Commerce Commission for the taxable period ended December 31, 1996 modified by an adjustment factor. The adjustment factor is a ratio the numerator of which is the average of the beginning and ending balances of the person's gross gas plant in service for the taxable period and the denominator of which is the average of the beginning and ending balances of the person's gross gas plant in service for the taxable period ended December 31, 1996, as set forth in the person's annual reports to the Illinois Commerce Commission for such taxable periods.
35 ILCS 615/2a.1