The effect of propylene concentration on NO oxidation as a function of temperature and position over a model Pt-Pd/Al2O3 diesel oxidation catalyst was investigated. Propylene had an apparent inhibition effect on NO oxidation. This apparent inhibition is a result of NO2, as the NO oxidation product, acting as an oxidant in the reaction with propylene. This was verified with experiments that included NO2, and a resulting significant temperature decrease in the onset of NO2 reduction when propylene was present. Furthermore, increasing amounts of propylene further decreased the NO2 reduction temperature. Similar results were observed with m-xylene and dodecane addition as well. The results also demonstrate that NO2 was consumed preferentially relative to O-2 during hydrocarbon oxidation. With low inlet levels of O-2, the addition of NO2 apparently inhibited propylene oxidation after the onset of NO2 reduction. This subsequent inhibition was due to the NO formed, demonstrating that propylene results in reduced NO2 outlet levels while NO inhibits propylene oxidation. (C) 2009 Elsevier B.V. All rights reserved.