The particle size distributions, morphologies, and chemical composition distributions of 14 coal fly ash (CFA) samples produced by the combustion of four western U.S. coals (two AA subbituminous, one lignite, and one bituminous) and three eastern U.S. coals (all bituminous) have been examined by computer-controlled scanning electron microscopy (CCSEM). The samples were obtained from a 50-kW combustor using an isokinetic sampler and separated aerodynami-cally), using a cyclone into PM fractions that were nominally <2.5 μm in mean diameter (PM2.5) and >2.5 mum in mean diameter (PM2.5,). However, the CCSEM particle size distribution results show that particles with mean diameter <2.5 μm typically constitute more than 80% of the total number of particles and particles with mean diameter >2.5 mum account for over 80% of the total particle volume for both the PM2.5 and the PM2.5+ fractions. The samples contained from 0.5 to 11.0 wt.% of unburned carbon and carbonaceous PM was the dominant form of PM <1 μm in mean size. Typically, over 80% of carbon-rich PM was < 1 mum and over 95% was <2.5 μm in mean size. The dominant carbonaceous PM morphology consisted of roughly spherical carbon particulates &SIM; 10-50 nm in diameter bonded together in complex chains and clusters that were usually < 1 mum in size. Glassy phases with rounded and frequently spherical morphologies were the dominant species observed for the inorganic PM. Particle compositions for several hundred inorganic particles are presented as ternary diagrams, which appear to be useful for source attribution. The inorganic PM derived from combustion of western low-rank coals, which are rich in carboxyl-bound Ca, is dominated by Ca-Si-Al glass phases, while that from eastern bituminous coals, which are rich in pyrite, is dominated by Fe-Si-Al glass phases. Important secondary signature glass phases that are also revealed by the ternary diagrams are Na-Si-Al for western CFA and K-Si-Al for eastern CFA. (C) 2004 Elsevier B.V All rights reserved.