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. First, the two-part equation predicts the emission amount from the percent fuel evaporated in an ASTM distillation at 160 F and the fuel’s Reid vapor pressure. Second, the reactivity per gram of the evaporative emissions can be determined from the percent C4 and C5 olefins and the percent C4 and C5 paraffins in the fuel.Utilization of the Evaporative Reactive Index (ERI) may be illustrated by the following examples. Removal of 65% of the butane from the typical Los Angeles gasoline would reduce the fuel evaporated at 160 F from 29 - 23% and lower Reid vapor pressure from 9.5 - 7 psi. The ERI equation would predict a decrease in evaporative emission amount from 110 - 68 gm/day and an increase in reactivity per gram (based on the NO2 formation rate scale) from 0.049 – 0.055. The ERI (NO2) would decrease from 5.4 - 3.7 (31%). On the other hand, replacement of the C4 and C5 olefins in the same fuel with C4 and C5 paraffins would not affect the percent evaporated at 160 F, Reid vapor pressure, or emission amount; but the equation would predict a decrease in reactivity per gram (NO2) from 0.049 - 0.028 and a decrease in the ERI (NO2) from 5.4 - 3.1 (43%).