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A diesel oxidation catalyst (Engelhard PTX Series) was evaluated on a medium-duty diesel engine (Caterpillar 3208, naturally aspirated, direct injection). Tests were conducted at six modes of the EPA 13 mode heavy-duty cycle to measure the total particulate, soluble organic fraction (SOF), sulfates, NO, NO2, NOx and hydrocarbons emitted by the engine with and without the oxidation catalysts. Chemical analysis of the SOF collected was carried out to determine the effects of the catalysts on each of the subfractions composing the SOF. The Ames Salmonella/microsome bioassay was employed to quantify the mutagenic properties of the particulate SOF. Test results show large increases in the amounts of total particulate and sulfate emissions due to the catalyst while the amounts of SOF are reduced by the catalyst. The amounts of NOx produced with and without the catalyst are similar, but the equivalent NO2 emitted with the catalyst installed is increased at most modes.