This paper presents results derived from Phase I of the Auto/Oil Air Quality Improvement Research Program. The Clean Air Act-defined mobile source toxic air pollutants benzene, 1,3-butadiene, formaldehyde and acetaldehyde have been measured in exhaust from twenty current model vehicles and fourteen older model vehicles during testing with 18 gasolines of varying composition.The gasoline fuel compositional variables evaluated included aromatic content, methyl tertiary-butyl ether (MTBE) content, olefin content, and the 90% distillation temperature (T90). The four fuel parameters were varied at target values of 45 and 20 vol % total aromatics, 0 and 15 vol % MTBE, 20 and 5 vol % total olefins and 360 and 280 °F 90% distillation temperature. An industry average fuel and an emissions certification test fuel were tested as reference fuels.In the current fleet, benzene levels were lowered when either fuel aromatics or T90 were reduced. Levels of 1,3-butadiene in the exhaust were lowered when MTBE was added and when olefins and T90 were reduced, but were raised by reducing fuel aromatics. Formaldehyde exhaust emissions were lowered when T90 was reduced but raised when aromatics were reduced or when MTBE was added. Acetaldehyde was lowered when T90 was reduced but was raised with a reduction in fuel aromatic content.Older fleet benzene emissions were lowered when aromatics were reduced and when MTBE was added. The emissions level of 1,3-butadiene was lowered when olefins and T90 were reduced. Formaldehyde was raised when aromatics were reduced. Acetaldehyde emissions increased as aromatics were reduced.The aggregate toxic emission levels for the 18 test fuels varied between 10 and 21 mg/mi for the current fleet and 27 and 40 mg/mi for the older fleet.