Mexican automobile manufacturers will begin to phase in catalytic converters on new models during the early 1990's. A more immediate concern that needs to be addressed is the large remaining older car population that will continue to pollute the air. Retrofitting these vehicles with catalytic converters is one of the alternatives being considered in a collective effort to improve Mexico City's air quality.In collaboration with Mexico City government officials, a vehicle demonstration was designed to show the benefits of an oxidation catalyst retrofit. This paper presents emission results of Mexico City's taxi fleet vehicles and displays the effectiveness of a Pt/Pd oxidation catalyst system in lowering emission levels. Two fleet vehicles were emission tested before and after catalyst retrofit modifications with the U.S. FTP-75 vehicle test method. Carburetor modifications were also included to study the effects of engine tuning on engine emissions and catalyst performance.A catalyst retrofit is one approach for achieving reductions in emissions. Prerequisites to a catalyst retrofit are the availability of lead free fuel and proper engine maintenance schedules.