The report describes three separate projects which relate to the study of the characteristics of automobile exhaust emissions. Part I studied the association of automotive fuel composition with exhaust reactivity. This involved experimental tests with different automotive engines and with gasolines of varied composition. Fuel composition was determined by gas chromatography. Exhaust reactivity was both estimated from detailed composition data and determined directly using a smog chamber. Part II, Toxic Products from Fuel Additives, studied procedures for the determination of any toxic gaseous products resulting from thermochemical reactions of fuel additives in the combustion process. The exhaust analyses included determination of: hydrocarbon, oxygenates, and the additive and its direct combustion products. Part III, Diesel Odor, studied the following problems: development of an odorant dilution and presentation system suitable for use with isolated exhaust components and synthetic odorants, establishing methods for demonstrating the reactivity and stability of diesel exhaust odorants, and isolation and collection of unstable components.