Individual particles from coal- and oil-fired power plants were analyzed by a scanning electron microscope equipped with an energy dispersive X-ray spectrometer to investigate the morphology and composition as a function of size. Samples were collected on filters by a dichotomous sampler in the fine (<2.5 micrometer aerodynamic diameter) and the coarse fractions (2.5 to 5-10 micrometers). In both fractions coal fly ash particles were predominantly smooth spheres, and no cenospheres (perforated hollow spheres) were detected. Almost 90% of the mass concentration occurred in the coarse fraction; the major elements included Al, Si, K, Ca, Ti, and Fe. Sulfur appeared as a surface layer on the mineral core. The abundances of Fe and S in each particle were highly variable. The ratio of Al to Si was fairly constant for most of the spheres but not for the relatively few Fe-rich or non-spherical coal fly ash particles. Over 90% of the mass of oil fly ash occurred in the fine fraction.