The report reviews information on the characteristics of solid and liquid fuels. Specifically, it deals with the chemical and physical characteristics of components of the fuel which are sources of sulfur, nitrogen, and trace element pollutants when that fuel is utilized. The review suggests that at least part of the sulfur and most of the nitrogen originate from compounds common to the fuels reviewed (coal, petroleum, tar sand oil, and shale oil). These are primarily organic sulfur and organic nitrogen compounds. For liquid fuels, it was concluded that intrinsic centers of sulfur and nitrogen contamination are found in the colloidal suspensions commonly known as asphaltenes and the more soluble resins. Trace elements are present as oil-soluble compounds in petroleum, tar sand oil, and shale oil. In coal, it was concluded that the nitrogen contaminants are present as organic compounds and that the sulfur is present both as organic and inorganic compounds. Trace elements in coal were categorized into those found principally with the organic matter of coal and those found present principally in the mineral matter associated with coal.