The emissions impact associated with increasing gasoline sulfur content was investigated using eight late-model vehicles, most of which were equipped with advanced emission control systems and certified as California Low-Emission Vehicles. The effect of returning to operation on low-sulfur fuel on emissions was also investigated. Vehicle testing was performed using California Phase 2 Certification test fuels with nominal sulfur levels of 40 and 540 ppm in combination with the LA4 and US06 driving cycles. In addition to exhaust emission measurements, engine-out emissions, air-fuel ratio, catalyst composition, and catalyst temperature data were collected. The data showed that most of the vehicles were sensitive to gasoline sulfur content as emissions increased when the vehicles were operated on the higher-sulfur test fuel; however, the degree of sensitivity varied from vehicle to vehicle. In addition, the data showed that the effects of operation on high-sulfur fuel were largely reversible for all pollutants following a return to operation on low-sulfur fuel on the LA4 driving cycle. Use of low-sulfur fuel with the more severe US06 driving cycle generally led to a more complete reversal of sulfur effects in those cases where complete recovery was not achieved on the LA4 cycle. Overall, the results of the study indicated that operation on high-sulfur gasoline did not result in permanent, adverse impacts on the emission performance of late-model vehicles. Although the study was not designed to evaluate the importance of factors related to vehicle design and operation with respect to sulfur sensitivity, a statistical analysis of the data suggested that differences in sulfur sensitivity may be related to differences in engine-out emissions and other factors.