Face to the difficulty to find an efficient catalyst in the reduction of NO by hydrocarbon in diesel exhaust gases, we have examined the interaction of some concerned reactant gases, NO, C3H6 and O-2, with a catalyst constituted of nanometer scale platinum particles deposited on alumina. Indeed, in order to understand the catalytic process, it seems essential to have information on the adsorption process and on the evolution of the structural parameters of the particles when submitted to such gases. This work has been performed on a 1% Pt/Al2O3 catalyst by using in situ EXAFS measurements. We have concentrated our study, on the one hand, on the influence of NO, C3H6 in the presence or not of an excess of oxygen and, on the other hand, on the influence of the total mixture. The most striking result is the growth of the platinum particles under NO. Moreover, when the catalyst is submitted either to a mixture of NO + O-2 or C3H6 + O-2, the particles are not fully oxidised. With the total mixture, the particles both grow and are oxidised.