A study conducted at the National Energy Technology Laboratory characterized twelve Class F fly ash samples from nine PC power plants in PA, WV, MD and TN, using scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). Both the surface and internal structure of fly ash particles were analyzed. All of the fly ash samples were comprised mainly of amorphous alumino-silicate spheres and a smaller amount of iron-rich spheres. The majority of the iron-rich spheres had two components: iron oxide and amorphous alumino-silicate. Both materials were apparent on the particle surface, and cross-sections clearly showed that the iron oxide and alumino-silicate were mixed throughout the fly ash particles. Calcium, the fourth most abundant element in the fly ash, was associated with oxygen, sulfur or phosphorous, not with silicon or aluminum. The calcium-rich material was distinct in both elemental composition and texture from the amorphous alumino-silicate spheres. The elemental concentrations, as determined by EDS, were consistent with ICP-OES and X-ray diffraction data.