Two fleets of thirty vehicles each were emissions tested in order to determine the effect of gasoline RVP reduction on tailpipe carbon monoxide (CO) emissions in Las Vegas and Los Angeles under conditions typical of winter CO exceedances in these two cities. The hypothesized emission reduction was confirmed for Las Vegas. However, for Los Angeles, the effect of RVP was questionable. The reason or reasons for this discrepancy between the two cities could not be completely resolved from this study. Detrimental emissions effects of reduced RVP under cold temperatures were found to be small and inconsequential.