The effects of fuel sulfur level on tailpipe emissions from advanced technology vehicles were explored in Phase 1 of a study conducted under the Petroleum Environmental Research Forum. In this study, fuels having three different levels of sulfur were tested according to the Federal Test Procedure (FTP) in low-mileage production vehicles designed primarily to meet California's Transitional Low Emission Vehicle (TLEV) standards. The three fuels consisted of a base gasoline containing 25 ppm by mass of sulfur, with the other two fuels created by doping the base fuel to 300 ppm and 600 ppm sulfur, respectively. Nine TLEVs and one Federal Tier 1 vehicle were tested at least twice on each fuel in an order that was balanced in time and randomized among the vehicles.Measured levels of non-methane hydrocarbons (NMHC) and carbon monoxide (CO) were found to decrease significantly when changing between the 300 ppm sulfur fuel and that containing 25 ppm sulfur, for all three modes of the FTP, and for the composite cycle. The relative changes in emissions were largest in the stabilized (Bag 2) mode, where the vehicles are fully warmed up and catalysts tend to be most active, and smallest during the cold start (Bag 1) mode. Changes in emissions observed between operation on the 300 ppm and 600 ppm sulfur fuels were small and not statistically significant. Differences in emission rates between individual vehicles were larger than the fuel sulfur effects measured in this program, with a factor of two or greater between the lowest and highest emitter for each pollutant. Different vehicles also appeared to have disparate responses to changes in fuel sulfur level, with one model that happens to use a palladium-containing exhaust catalyst appearing more sensitive to this parameter than the other models, which all employ platinum-rhodium catalysts. Nonetheless, the observed model-mean emission rates for the test fleet were much lower than those observed for 1990-technology test fleets, and appear to be within the TLEV emission standards even when operated on the fuel containing 600 ppm sulfur.