A study of the lean NOx reduction activity with propene in the presence of water over Ag/Al2O3 catalysts with different silver loadings (1.5-6 wt.%), prepared by precipitation from inverse microemulsions, has been performed. The highest NOx reduction activities were; observed for catalysts with intermediate silver loadings. Complementary in situ-DRIFTS results on one of the active catalysts indicate formation of surface adsorbed acrylate and nitrate species resulting from oxidation of propene and NO, and of isocyanate and cyanide species, which are final intermediates in the NOx reduction process. Comparison by in situ-DRIFTS between one of the active catalysts (with 1.5 wt.% Ag) and an inactive one (with 6 wt.% Ag) reveals that a certain nitrate concentration is maintained under reaction conditions at the surface of active catalysts. This is attributed to the existence of different NOx reaction paths, involving interaction of activated NOx species with acidic hydroxyls of the alumina surface for active catalysts. Characterisation results by XRD and TEM indicate that amorphous silver-containing phases which are active for NOx reduction lie over a wide range of particle sizes, while the different size of these entities before and after reaction indicates significant mobility under reaction conditions.