Diesel particulate emissions continue to challenge researchers and scientists in the industrialized world. ( 1 ) * In spite of many design improvements in recent years, diesel engines are still emitting particulate emissions which are uncomfortably close to the mandated levels.(2) Engineers have been working on solutions involving exhaust aftertreatment.(3) Two solutions were subjects of intense research since the mid-seventies, and appear to be ready for serial production at this time. One of these two solutions is the diesel particulate trap, while the other is the catalytic converter.(4) Traps are usually more effective with insoluble particulate, while catalysts reduce organic particulate. Detailed analysis of exhaust particulate for a given engine should assist in selecting the most effective method of aftertreatment for that application.A medium-duty diesel engine was mapped with special emphasis on determining the particulate matter constituents under various operating regimes. Catalytic aftertreatment was also applied to further determine where this control method was most effective. Results of this work are discussed in this paper.