Kaolinite, illite, and a random mixed layer illite-montmorillonite were observed in nearly every sample of clay minerals obtained from Illinois coals by an electronic low temperature ashing technique. Montmorillonite, as a separate phase, was observed only rarely and chlorate was found in but one sample. Authigenic kaolinite was commonly observed in all of the coals studied. The technique uses a radio frequency discharge to produce activated oxygen which ashes the coal at temperatures below 190 degrees C. The clay mineral assemblages in Herrin (No. 6) Coal in the southwestern part of the Illinois Basin contain more mixed layer clay minerals relative to illite than do those in the southeastern part of the basin. A similar decrease of illite from east to west, and a relative increase in mixed layer material occurs in the clay minerals of the underclay of this coal. The distribution of clay minerals with the exception of authigenic kaolinite, is thought to be primarily the result of the introduction of fine grained sediments from the east and northeast and the reaction of the minerals with the coal swamp environment.