Information from the literature and from on-going test programs (government and industry) was analyzed with regard to the effect of gasoline Reid vapor pressure (RVP) on total vehicle hydrocarbon (HC) emissions, including evaporative, refueling, and exhaust emissions. A reduction in the average RVP of summer gasolines from present commercial levels to 9 psi was estimated to decrease total vehicle hydrocarbon emissions by 9-25 percent. With such reductions, hydrocarbon emission inventories for three major cities (Detroit, New York, and Dallas) would be decreased by 3-7 percent and, consequently, local ambient ozone levels would be reduced as much as 9 ppb. Accordingly, in many areas of the country, RVP reduction could make an important contribution toward achievement of the National Ambient Air Quality Standard (NAAQS) of 120 ppb ozone.Gasoline vapor pressure reduction to 9 RVP would provide slightly greater control of total vehicle HC emissions than would service station (Stage II) or on-board control of refueling emissions. More importantly, RVP control would provide immediate emissions reductions for the entire vehicle population (rather than a gradual phase-in of that benefit tied either to new vehicle sales and scrapping of old cars or to installation of Stage II equipment). Although low RVP gasoline might cost about 1 percent more than present summer gasoline, RVP control would be competitive with on-board or Stage II control on a cost-effectiveness basis.New data confirm that cold driveability at intermediate temperatures would not be impaired by control of summer-gasoline RVP to 9 psi.