Most major metropolitan areas in Mexico have continually violated government health standards for ozone. Throughout this period policy plans have been developed for lowering urban ozone, and bringing cities into compliance with the law, among them, fuel reformulation. In this work, 12 fuels were prepared in several sets, to study the ozone-forming potential effects of individual fuel properties of Tier 0 and Tier 1 in-use vehicles of the Metropolitan Area of Mexico City. The results indicate that the specific reactivities for Tier 1 vehicles are in general much higher than those reported in similar studies performed by other groups. An emission inventory for the 2010 year was calculated for each fuel in order to provide different scenarios for future formulations. (c) 2006 Elsevier Ltd. All rights reserved.