A test apparatus containing two raw-gas particulate sampling systems was designed and developed to determine accurately the performance of various materials as diesel exhaust particulate filters. This apparatus was used in conjunction with a single-cylinder diesel engine and a series of fibrous filters to quantify experimentally the effects of filter face area, filter thickness, packing density and fiber diameter on filter performance. Time-dependent efficiency measurements made with this system were smooth and consistent for all of the filters tested and correctly reflected the changes made in these key trap parameters. In all cases, the collection efficiency of the filters increased during the first hour of operation and then reached a relatively constant level. The pressure drop across the filters increased continuously as particulate material accumulated on the filters. Particulate size-distribution measurements made upstream and downstream of an exhaust filter showed that the smallest exhaust particles were more efficiently captured than the larger particles.