Olefins are among the most photochemically reactive components of automotive hydrocarbon emissions. Because of their reactivity, the California Air Resources Board (CARB) has proposed limiting total olefins in reformulated gasolines to levels as low as 4-6 vol% as one means of controlling the formation of ground-level ozone. However, the Clean Air Act Amendments of 1990 have no specific provisions for limiting reactivity of emissions.Limiting total gasoline olefins would be very costly to the refining industry. However, removal of light olefins (C5-, see NOMENCLATURE for definition) only would be less expensive and would have a less severe impact on the gasoline octane pool. To determine how olefin carbon number affects the atmospheric reactivity of automotive evaporative and exhaust hydrocarbon emissions, speciated emissions data from Phase I of the joint Auto/Oil Air Quality Improvement Research Program (AQIRP)(1) were analyzed with a new mathematical model. Removal of individual olefins was simulated by systematically setting their concentrations in the fuels to zero, adding back corresponding amounts of appropriate paraffins to replace them, and calculating exhaust, hot soak, and diurnal emissions reactivity with the Carter Maximum Incremental Reactivity (MIR) scale.Modeling results show that, if all olefins were removed from industry average gasoline (Auto/Oil Fuel A), total ozone forming potential (OFP, grams of ozone per mile traveled) would be decreased by about 12%. Removing C5 olefins alone accounts for about 70% of this reduction. From the emissions data analyzed, we have concluded that the incremental benefit of removing olefins decreases by a factor of three to four for each increase in the carbon number of the olefins removed in the range of C5 through C8.An experimental exhaust emissions study was also completed that tested 9 fuels in a 10-vehicle fleet. Criteria emissions were determined by the Federal Test Procedure with full speciation of hydrocarbons. Results indicated that olefins constituted only 14-20% of total mass nonmethane organic gas (NMOG) emissions but 29-49% of the total OFP. The C5- olefin fraction accounted for more than 94% of the total olefin contribution to OFP. This paper demonstrates that the environmental benefits of removing olefins become negligible as olefin carbon number increases above five.