The increased importance of diesels has caused a significant amount of discussion as to the ability of the manufacturers to meet the recently announced EPA Diesel Emission standards. Since there was little or no experience with the effects of installing an oxidation catalyst in a Diesel-powered vehicle, the EPA decided to investigate this configuration for its effects on both regulated and unregulated pollutants. Due to the relatively low hydrocarbon and carbon monoxide emissions typical of Diesel exhaust the initial question was whether the catalyst would attain a sufficient temperature to allow it to function as intended. The conclusions drawn from this EPA evaluation test can be considered to be qualitatively and quantitatively valid only for the specific vehicle and catalyst used. After establishing that the catalyst would function, its effect on emissions, both gaseous and particulate, on fuel economy and on vehicle performance were to be investigated.