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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.