The passive oxidation of particulate matter (PM) in a diesel catalyzed particulate filter (CPF) was investigated in a series of experiments performed on two engines. A total of ten tests were completed on a 2002 Cummins 246 kW (330 hp) ISM and a 2007 Cummins 272 kW (365 hp) ISL. Five tests were performed on each engine to determine if using engine technologies certified to different emissions regulations has an impact on the passive oxidation characteristics of the PM.A new experimental procedure for passive oxidation testing was developed and implemented for the experiments. In order to investigate the parameters of interest, the engines were initially operated at a steady state loading condition where the PM concentrations, flow rates, and temperatures were such that the accumulation of PM within the CPF was obtained in a controlled manner. This engine operating condition was maintained until a CPF PM loading of 2.2 ±0.2 g/L was obtained. The engine operating conditions were then changed in order to vary the parameters of interest and perform the passive oxidation stage of the test.The test matrix was designed to concentrate on the variables that most affect passive oxidation of PM within the CPF. These variables include NO₂ and NO concentrations into the CPF, the NO₂/PM and NOx/PM ratios at the inlet of the CPF, exhaust temperature, and the exhaust flow rate. The test matrix provided for a wide range of conditions including average CPF temperatures from 260 to 460°C, NO₂/PM mass ratios from 3 to 59, NOx/PM mass ratios from 8 to 160, and exhaust mass flow rates of 5.6 to 18.0 kg/min. By gaining a better understanding of how these variables affect passive oxidation, engine operating conditions can be selected that optimize the use of this regeneration method. This will reduce the dependency on active regenerations to clean the filter and the fuel penalty associated with this strategy.Experimental results during the loading stage of the tests demonstrated that the PM generated by each engine under similar operating conditions had similar oxidation rates with different NO₂ and NO concentrations at the CPF inlet. Results from the passive oxidation portion of the experiments show a strong correlation between the reaction rate and the CPF average temperature. When performing a mass balance across the CPF, calculations for some of the experiments indicate that at temperatures less than 400°C, assuming that PM oxidation is only accomplished by NO₂ oxidizing the PM, more NO₂ is consumed in the CPF than what is available at the CPF inlet. The hypothesis was that back diffusion of NO₂ from the catalyst on the CPF substrate wall occurs. Calculations of the Péclet number within the CPF showed that the transport of NO₂ is dominated by diffusion under the operating conditions in these experiments. This finding indicates that additional NO₂ could be back diffusing into the PM cake layer after NO is oxidized into NO₂ at the CPF catalyst surface which would then account for the additional PM oxidation that occurred in the CPF.