Concentrations of 42 minor and trace elements in four size fractions of stack fly ash from a large Western coal-fired power plant are reported. Mass median diameters of the four size fractions were 2.4,3.7, 5.0, and 18.5mum. Based on the enrichments relative to coal as a function of fly-ash particle size, the elements are grouped into three classes: Group I elements that show little or no enrichment in the small particle fraction (Al, Ca, Cs, Fe, Hf, K, Mg, Mn, Na, Rb, Sc, Ta, Th, Ti and the rare earths: Ce, Dy, Eu, La, Nd, Sm, Tb, and Yb); Group II, elements whose enrichments increase with decreasing particle size (As, Cd, Ga, Mo Pb, Sb, Se, W and Zn); and Group III, elements whose behavior is intermediate to that of elements in Group I and II (Ba, Be, Co, Cr, Cu, Ni, Sr, U, V). The lithophilic and chalophilic geochemical behavior, respectively. The chemical behavior of individual elements in Group III is discussed.