Diesel emission control is being addressed worldwide to help preserve the global environment.In 1994, emission controls in the U.S. called for reduction of diesel particulate matter (PM) to 10 to 20% of 1986's initial limit. In the same year, we developed and marketed small and medium duty trucks which were equipped with PM reduction systems that oxidize soluble organic fraction (SOF) contained in the PM, in order to satisfy these new regulations.Prior to their marketing, a catalyst was selected from among several types of candidate catalysts. Durability tests were performed using a catalytic converter-equipped small duty truck to verify the durability of the chosen catalyst. The durability test course was set up combining urban areas and expressways in the southern part of California, U.S.A.. The cumulative total distance covered on the test course reached 200,000 km.During the durability test, the catalyst was evaluated by measurement of PM emission using a chassis dynamometer. After the durability tests, the catalyst was taken out of the converter muffler. Then microstructural changes in the catalyst were studied by use of various analytical methods.The results of these tests convinced us of catalyst's durability and ability to maintain SOF reduction.