Coal fly ashes from different sources were extensively studied. Fly ash consists of as many as 12 different particle types, made up mainly of Si, Al, Ca, K, Na, and Fe. Many p potentially volatile trace elements (As, Se, Ga, In, Pb, Cd) are always preferentially associated with more respirable particles. Many other minor and trace elements may also show an inverse particle size dependence, depending on the coal fly ash. Elements showing no dependence include Al, Ba, Ca, Co, Fe, K, Mg, Rb, Sc, Si, Ta, Ti, and the rare earths. Many of the more volatile and potentially hazardous trace elements are surface associated. Most potentially hazardous inorganic species in fly ash can be leached into aqueous solution but are unlikely to give rise to solution concentrations of great concern. The physical and chemical characteristics and behavior of polycyclic organic matter (POM) associated with coal fly ash was also studied. Theoretical models show that adsorption of POM on coal fly ash will occur in seconds in a typical emission stream and that the process is highly temperature dependent. Surface-adsorbed POM is highly reactive to some nitrogen and sulfur oxides at levels expected to be found in power plumes. It was found that some POM absorbed on fly ash are stabilized aganist photochemical decomposition, while other compounds decompose readily upon adsorption. Although fly ash emitted to the atmosphere contains much more POM than precipitator ash, there is an insufficient amount to make a discernible impact on any terrestrial aquatic system.