The concentration of 21 elements in fly ash from three boilers (75 MW, 150 MW, and 300 MW) at the EGAT power plant, Mae Moh, Thailand, were determined by instrumental neutron activation analysis (INAA). The concentration of 10 major elements was determined by x-ray fluorescence (XRF). Arsenic, Co, Cr, Ni, Mo, and Sb generally increase in concentration going from bottom ash (BA) through the sequence of electrostatic precipitator ashes (ESPA) and reach maxima of As (352 ppm), Co (45 ppm), Cr (105) ppm, Mo (32 ppm), Ni (106 ppm), and Sb (15 ppm) in the ESPA. Cerium, Cs, Fe, Hf, La, Sc, Ta, Tb, and Yb did not exhibit concentration trends or are variable except in the case of one boiler (unit 6), which showed an increase going from BA to ESPA. Only Br decreased in composition going from BA to ESPA. Rubidium, Sm, U, and Th showed marked variation in trends. Scanning electron microscopy coupled with energy dispersive spectroscopy (SEM/EDS) was used to identify element assemblages within discrete particles and particle morphology. Comparison of fly-ash morphology shows that the BA is composed of large angular shards/fragments with only a small amount of semi-spherical particles. Successive ESPA is composed mainly of spherical particles which decrease in average diameter with increasing distance from the boiler. The major elements identified by EDS were Al, Si, S, K, Ca, Fe, and Ba, with minor amounts of Mg, Na, Ti, Mn, and Sr. Aluminum, Si, K, and Ca occur together and are present in most of the fly-ash particles. Barium was found as a major component with Ca, Al, and Si. Iron and Ca are usually associated with sulfur. Some small spheres (<5 mum) are comprised almost entirely of Fe (probably as oxide). Symplectite textures are noted in high-Fe phases. All elements except Br are significantly enriched in the fly ash relative to the coal, which contains 35% ash. Particle chemistry is consistent with the major mineral phases identified by x-ray diffraction (XRD), which include: quartz, magnetite, mullite, gehlenite, anorthite, hematite, anhydrite, and clinopyroxene.