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The effect of fuel composition on automotive evaporative emissions has been studied using five cars, not equipped with evaporative emission controls, and a total of 31 fuels. The amount of evaporative emissions increased with increasing fuel volatility. Also, the evaporative emission photochemical reactivity per gram increased with increasing C4 and C5 olefins in the fuel and decreased with increasing C4 and C5 paraffins. For an assessment of the smog potential of evaporative emissions, the amount and reactivity per gram should not be considered independently, since they both are simultaneously dependent on fuel composition. The product of amount and reactivity per gram (the Evaporative Reactive Index) is a good measure of the contribution of evaporative emissions to photochemical air pollution. An empirical equation for predicting the Evaporative Reactive Index from fuel properties has been derived.