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The performance of diesel particulate filters (DPF) has historically been evaluated by gravimetric efficiency, which measures the mass of particulate matter (PM) trapped in the filters. This method does not measure the filtration efficiency at different PM size ranges and, therefore, cannot provide information on fractional performance of DPFs. This fact becomes significant because the adverse effects of diesel PM emissions on human health and the environment are size dependent. A previous study investigated the fractional performance of DPFs under steady state conditions using a scanning mobility particle sizer (SMPS). In the real world, however, nearly all engines operate under transient conditions. DPFs also have to perform under such conditions, so any measurement of performance must be able to react to quickly changing DPF conditions.