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The combustion efficiencies of diesel oxidation catalysts (DOC) targeted for use in the heat-up role of active diesel particulate filter (DPF) systems are investigated. Light-off tests using synthetic gases and fuel injection studies on light and heavy duty engines, both before and after thermal aging, are carried out. These evaluations are used to demonstrate differences in activity between closely related Pt-only and Pt/Pd formulations. Post-mortem analyses are conducted to determine the basis for the performance differences observed during the fuel injection studies. These analyses include measurement of the accumulation of carbonaceous compounds on catalyst surfaces which are associated with incomplete combustion of the injected fuel. Aging cycles developed for DOC+DPF systems incorporating heat-up by in-exhaust fuel injection on heavy-duty diesel engines are presented. The impact of these aging cycles on the performance of a Pt/Pd catalyst are summarized.