Mathematical modeling of ambient air photochemistry requires comprehensive mobile source hydrocarbon emissions speciation. Passenger car tailpipe and evaporative hydrocarbon emissions have been examined using procedures described in the Federal Register for emissions certification. Hydrocarbon emission rates and compositions were determined for four passenger cars: a 1963 Chevrolet, a 1977 Ford Mustang II, a 1978 Mercury Monarch, and a 1979 Ford LTD-II. These vehicles are representative of a wide range of exhaust and evaporative emissions control configurations. Both emission rates and compositions were dependent on the emissions control devices used with the vehicles, and the fuel composition and vapor pressure. Generally, tailpipe catalyst control systems removed unsaturated olefinic, aromatic, and acetylenic hydrocarbons to a greater extent than saturated paraffinic hydrocarbons. The impact of evaporative control systems on composition appeared dependent on the aromatic content of the fuel with little change observed on emissions reduction for low aromatic fuels and an apparent increase in the relative abundance of aromatic compounds on emissions reduction for high aromatic fuels. The emission rate of benzene, emphasized because of its potential carcinogenicity, was sensitive to both fuel benzene and total aromatic content.