This study examined the emissions characteristics of non-tampered, in-use vehicles when operated on premium fuels at seasonal temperatures typical of the Northeast United States. Two test vehicles representing different emission control systems were tested: a 1980 carburetion vehicle with an oxidation catalyst (C/OC), and a 1985 port fuel injection (PFI) vehicle with closed loop three way catalyst (TWC). The tests were performed at low and intermediate temperatures (20°F,35°F,50°F and 70°F) with premium fuels blended to meet volatility specifications which corresponded to these seasonal temperatures. Fuels containing methyl tertiary butyl ether (MTBE) and ethanol were included to investigate the relative effect of these oxygenates on exhaust emissions at different temperatures.Results from the exhaust emissions tests showed that exhaust hydrocarbon and carbon monoxide emissions of the C/OC vehicle increased as temperature decreased. Below 50°F, the PFI/TWC vehicle also exhibited an increase in exhaust emissions with a decrease in test temperature. The temperature effect on exhaust emissions was greater than the differences due to fuel vapor pressure variances and/or fuel oxygen content. At higher temperatures, 50°F and 70°F, increased fuel volatility produced a slight increase in carbon monoxide emissions of both vehicles. Generally, the vehicles showed a downward trend in carbon monoxide emissions from fuel oxygen. Diurnal evaporative emissions increased with an increase in the fuel vapor pressure at the test temperature.