The H-2/NO/O-2 reaction under lean-burn conditions has been studied by means of in situ DRIFTS, reactor measurements and temperature-programmed desorption with the aim of understanding the very different behavior of Pd/TiO2 and Pd/Al2O3 catalysts. The former deliver very high NOx conversions (70-80%) with good N-2 selectivity whereas the latter show very low activity. In addition, Pd/TiO2 exhibits two distinct NOx reduction pathways, thus greatly extending the useful temperature range. It is shown that the Pd/TiO2 low-temperature channel involves adsorption and subsequent dissociation of NO on reduced (Pd-0) metal sites. The low activity of Pd/Al2O3 is a consequence of palladium remaining in an oxidized state under reaction conditions. The high-temperature NO reduction channel found with Pd/TiO2 is associated with the generation and subsequent reaction of NHx species.