Emissions from automobiles have long been considered a prime source of pollutants involved in smog formation and ozone production. The reactive potential of the species emitted has been studied extensively, and many reactivity schemes have been proposed. Most of the data on the detailed composition of the emissions from automobiles were taken from new or prototype vehicles. The study was undertaken to ascertain the mass and the detailed hydrocarbon and aldehyde composition of emissions from vehicles actually driven by the public. Forty-six vehicles, 1975 through 1982 models, were tested by using the Federal Test Procedure driving cycle, the Hot Soak Evaporative Test, the New York City driving cycle, and the Crowded Urban Expressway driving cycle, also known as the Sulfate Cycle. Overall composition was quite consistent among cycles and years, with some changes occurring in the 1981 and 1982 models. Mass emissions decreased with model year, showing the most significant decreases in latter years as the standards became more stringent. Eighty-two individual hydrocarbons and 10 aldehydes are reported for each test condition. The ratio of hydrocarbons to oxides of nitrogen increased dramatically on the lower speed cycles.