An experimental study was conducted to characterize the aerosol that passes through a wall-flow monolith trap and to determine how the trap performance varies with engine operating condition.Steady-state experiments conducted with a 5.7-L diesel engine operating at several test conditions showed that the wall-flow trap collected nearly all of the carbonaceous particulate material, regardless of the operating condition. However, the ability of the trap to collect the volatile particulate material was dependent upon the exhaust gas temperature.Particle size measurements showed that significant concentrations of small, nuclei-mode particles exist downstream of the filter at some engine operating conditions. The quantity of nuclei-mode particles was shown to be dependent primarily upon exhaust gas temperature. Without the exhaust filter, the concentration of these small particles was generally negligible. These small particles are of particular interest since they are capable of penetrating deeply into the human pulmonary system.