C3H6 oxidation over a Pt/Al2O3 catalyst with or without NOx present was investigated, and the reaction mechanism was studied by diffuse reflectance infrared spectroscopy (DRIFTS). C3H6 oxidation is inhibited by the presence of NO, and vice versa, and data indicate that adsorbed NOx can react with gas phase C3H6. DRIFTS results confirm the reaction between C3H6 and nitrates, which are formed during NOx adsorption, with linear nitrites observed as reaction products. Therefore, a reaction route is proposed for C3H6 oxidation in the presence of NOx, namely, nitrates acting as oxidants. Using NO2 instead of NO, or using a high NOx/C3H6 ratio, which is beneficial for nitrate formation, favors this nitrate reaction pathway.