Scanning electron microscope/electron microprobe chemical analyses of individual grains of several western fly ashes have shown an inter-grain variation in composition for ashes derived from both lignite and subbituminous western coals. SRM 2689 (a Class F fly ash from a bituminous coal) and SRM 2691 (a Class C fly ash from a subbituminous coal) have been included in the study and also show inter-grain chemical variation. A classification scheme of fly ash grain compositions has been developed and has proven to be useful in illustrating the differences and similarities of the ashes. There is a positive correlation, for example, between grains rich in SiO2 and Al2O3 and the Na2O content of the grains in lignite and subbituminous coal derived fly ashes. There is also a positive correlation between grains rich in CaO and the MgO content of the grains in both classes of fly ashes.