Results of a study designed to construct a geothermometer based upon thermal alterations in the organic constituents of Hanna No. 1 coal lying within the Paleocene Hanna Formation and carbonaceous shale samples are presented. The samples were observed microscopically and determinations were made as to the maximum temperature to which the sediments has been subjected. The organic constituents of the samples (macerals) were observed described, and quantified as changes occurred in their appearance during heating at progressively higher temperatures. Direct petrographic changes were noted to take place in the Hanna No. 1 samples during heating. Decreasing abundances of fluorescence macerals, increasing vesicularity and cracking in vitrinites, and eventual elimination of all organic constituents except vitrinite could be correlated with temperature ranges. The liptinites reacted earlier than the vitrinites, and this information could have important implications as far as optimum gas composition and production are concerned. (BLM)