Sodium promotion strongly enhances both the activity and the nitrogen selectivity of Rh/gamma-Al2O3 catalysts in the reduction of NO by propene. The optimally promoted catalyst (7.3% w/w Na) exhibits a similar to 3 times higher TOF and 90% N-2 selectivity (at 375 degrees C) as compared to 53% for the unpromoted catalyst under the same conditions. Na also suppressed the formation of CO and HCN. At sufficiently high Na loadings the system exhibits poisoning. The improved performance and kinetic behaviour are understandable in terms of Na-induced enhancement of NO adsorption and dissociation on the Rh surface, in competition with propene and its decomposition products.