During selective reduction of NOx under lean-burn conditions, a Pt particle size dependency has previously been observed with various supports. In this study, we have examined the influence of various parameters over a large range of initial metal dispersion: nature of the support (silica or alumina), support porosity, presence of impurities (particularly chlorine or sulfur), nature of the platinum precursor salt, and Pt particle size distribution. Furthermore, we have considered the mean particle size after sintering under the reactant mixture up to 773 K. Of the factors considered, only the Pt dispersion is of key importance. The intrinsic activity increases with decreasing dispersion (measured initially or after reaction) for each of the main reactions: reduction of NO into N-2 or N2O, oxidation of NO into NO2, or oxidation of C3H6 into CO2. The dispersion does not clearly affect the selectivity. (C) 2000 Academic Press.