In view of increased concerns regarding the effects of diesel engine particulate and NO, emissions on human health and the environment, legislators are currently reviewing and proposing legislation targeting the reduction of these pollutants. The reported serious health risks of particulate matter on the respiratory system and its carcinogenic effects, along with the known contributions of NO, in acid rain and ground ozone formation, demand that the enacted legislation reflect in severity the health and environmental threats. As a consequence, diesel engine manufacturers and users are under increasing pressure to greatly reduce the engine’s exhaust emissions. A system which is currently being proposed for the simultaneous control of diesel particulate matter and NO, emissions involves the use of a cerium fuel-borne catalyst/filter/EGR system. This paper describes the principles of operation of Rhone-Poulenc’s cerium fuel-borne catalyst and the factors that govern its use.