Air quality in Santiago (4.500.000 habs) has deteriorated during the last decade. This situation is aggravated by the topographical and meteorological set up of Santiago. Around 55% of the trips are served by diesel buses, the rest through, cars, taxis, shared taxis, metro and foot. Buses in Santiago are constructed using truck chassis with school bus type body. Engines are water refrigerated, 4-6 cilinder direct injection diesels. Analysis of environmental data shows that 70% of PM-10 comes from diesel emissions. Emissions have been measured according Santiago's cycle. Measurement procedures EPA based. Emissions are 3 gr/km.I&M procedures were researched to determine the motives for the high emissions. Results showed that this area is highly underdeveloped. A pilot program in which different fuels and technologies were used was implemented. Results of this experience is that the best C/E solution was well maintained diesel engines. Emissions of 0,34 gr/km was achieved. Other aspects analyized were: emission inspection through dynamometer and opacimeter. This system is being automatized. New emission standards were drafted. The standards establish a gradual increase in emission limitations. In the final step 1991 EPA diesel standard for urban buses are proposed.