Reduction of nitrogen oxides under oxidative atmosphere is a very extensively studied reaction, but it is still very difficult to understand and to follow the various pathways of the reaction. Two alumina supported 0.2 wt% platinum catalysts, reduced by hydrogen in an oven heated either by microwave irradiations or by Joule effect, with different metal dispersion of 60% and 90%, respectively, were studied. By the use of labelled compounds we were able to show the presence of an exchange reaction between (NO)-N-15-O-16 and (NO)-N-15-O-18 which occurred on both catalysts. It means that [(NOO)-N-15-O-16-O-18] is the intermediate species. Such product, (NO)-N-15-O-18, is less formed on the microwave catalyst "MW" than on the classical one "CT". Experiments were performed at 22 and 550 Torr, between 150 degrees C and 250 degrees C. Near atmospheric pressure, "MW" catalyst gives higher initial rates for N-15(2) formation than the "CT" catalysts. At low temperature, the nitrogen selectivity is higher on "MW" catalyst than on the other catalyst, From the apparent activation energy values, one may deduce that several mechanisms are responsible for the N-15(2) formation depending on the reaction temperature and the catalyst used. On the 0.2% Pt/Al2O3 "CT" catalyst, an additive process between propene and (NO)-N-15 takes place at low temperature giving nitroso and oxime intermediate species. At high temperature, a partial oxidation of propene occurs, giving a ketone, before the (NO)-N-15 reduction to N-15(2) With this catalyst only two sites with different activity an involved. On the 0.2% Pt/Al2O3 "MW" catalyst the reactants are seated on three sites with different activity. This particularity reinforce the proposals concerning the "MW" catalyst which may exhibit particular shapes for the aggregates having different crystallographic orientations. What is surprising, for this "MW" catalyst, is the fact that we already observed a specific reactivity under reductive atmosphere in reforming reactions [J.M. Thiebaut, G. Roussy, M.S. Medjram, F. Garin, L. Seyfried, G. Maire, Catal. Lett. 21(1993) 133; L. Seyfried, F. Garin, G. Maire, J.M. Thiebaut, G. Roussy, J. Catal. 148 (1994) 281] and now its behaviour is also particular under oxidative atmosphere.