Samples of coal were obtained from the Hanna No. 1 coal at DOE’s Hanna underground site. Each sample was heated in a helium atmosphere for one hour, with alteration temperatures beginning at about 23 degrees Centigrade, and increasing at 50 degree increments between 50 degrees and 600 degrees Centigrade. The process is believed to have duplicated conditions at certain locations within an underground coal gasification reactor. A method of determining gasification reaction temperatures by relating visible changes in coal macerals with known temperatures of alteration was developed. Macerals were identified using white and blue-ultra violet incident light. A semi-mobile fluorescing maceral, exudatinite, was found to be unusually abundant in all samples and displayed a steadily decreasing abundance as heating progressed. Significant changes began between 250-300 degrees Centigrade when reaction vesicles developed due to exudatinite and exinite destruction. Above that temperature, increased cracking and vesicularization of the coal matrix was accompanied by extinction of fluorescence at 400 degrees Centigrade. The devolatization of exudatinite and other Liptinites is among the earliest of conversing reactions. Vitrinite reflectivity increased as temperatures increased.