Light duty gasoline vehicles account for most of CO, hydrocarbons and NOx emissions to the urban environment in Mexico City. From the beginning of year 2001, vehicles comply with Tier I emissions standards. Two types of Regular gasoline are available in the country. In those ozone non-attainment areas Regular oxygenated gasoline with an average of 450 ppm S is available, while on the rest of the country, a gasoline of 750 ppm S, is sold. It is anticipated then that many or all Tier I vehicles in ozone non-attainment zones would be occasionally or regularly fueled with high sulfur gasoline. In this work we evaluate the impact of sulfur on new, and in-use vehicles model years 1993-2001 with gasoline ranging from 90 to 815 ppm S. It is interesting to note that for Tier 0 vehicles, CO emissions are above the standard limits at all sulfur levels, while Tier I vehicles are very close to the limits for NOx emissions. (C) 2003 Elsevier Science Ltd. All rights reserved.