Certification of particulate emissions from diesel motor vehicles involves filtration of measured aliquots of the total air diluted exhaust. Seven commercially available filter media were examined for this purpose. The media included a variety of PTFE membrane filters, glass fiber filters, and PTFE coated glass fiber filters. Relative flow resistance (pressure drop), collection efficiency, and gas phase adsorption were examined. Filter structural differences, which influence particulate collection mechanisms, sample flow rates and pressure drops, were studied microscopically. Two media, a membrane and a fiber filter, were also examined microscopically with varying levels of particulate load to determine the role of collected particles on the filtration of subsequent particles. The results obtained indicate that under the defined test conditions the membrane filters yield low gram per mile emissions rates due to difficulties with collection of Federal Test Procedure phase 1 emissions, and the Gelman A-E glass fiber filter high rates due to adsorption of gas phase emissions. The mechanics of particle collection are similar for both membrane and fiber filters as applied. Diffusion deposition is important with fiber filters for about 5 percent of the collected particulate matter, the remaining 95 percent collected primarily by direct interception. With the membrane filters direct interception is the dominate process.