The U.S. Bureau of Mines has sponsored research Co determine the particle size distribution and concentration of submicron particles upstream and downstream of a ceramic particle trap mounted in the exhaust stream of a Caterpillar 3304 diesel engine. Particle size distribution and mass were measured with an electrical aerosol analyzer, a diffusion battery-condensation nuclei counter combination, and filters. The engine was operated at 1400 and 1800 RPM and 3 load conditions at each speed-In general, the collection efficiency of the trap was high, ranging between 89 to 96%. Size distribution analysis revealed that the trap was generally more efficient at removing particles smaller than 0.1 μm diameter than larger particles. However, under certain conditions formation of nuclei (less than 0.056 μm diameter) downstream of the trap took place. This is probably associated with the penetration of hydrocarbon vapors through the the trap which nucleate to form very fine particles as the exhaust is diluted and cooled. Particle concentration measurements during regeneration revealed significant transient particle generation. A large concentration peak of accumulation mode size particles (0.056 to 1.0 μm diameter) appeared shortly after the onset of regeneration, followed by a broader peak of nuclei mode size particles (0.01 to 0.056 μm diameter). It is believed that the accumulation mode particles result from oxidation and breakup of particles deposited on the trap while nuclei mode particles result from nucleation of hydrocarbon vapors driven from the trap during regeneration.