Abstract The reduction of nitric oxide with hydrogen was studied over a Pt 0.25-Rh 0.75(100) alloy surface used as a model catalyst for the automotive three-way catalyst. This paper emphasizes the mechanisms of the different reactions leading to the products dinitrogen, ammonia and nitrous oxide. For this purpose the reaction was studied under various experimental conditions including reactivity measurements both in the 10 −7 mbar range under steady-state conditions and in the 10 mbar range with varying NO/H 2 ratio. In addition, the thermal decomposition of NO and the reactions of NO + NH 3 were investigated. 15NO and 15NH 3 were used in order to gather additional information concerning the mechanisms of the formation reactions of the various N-containing products. The surface was characterized by using low-energy electron diffraction. Auger electron spectroscopy and thermal desorption spectroscopy. The main conclusions emerging from these studies are: (a) N 2 can be formed by combination of 2 N adatoms in the whole temperature range used (350–1300 K) provided that sufficient N adatoms are available. (b) Below 600 K the main contribution to N 2 formation is via NO ads + N ads → N 2+ O ads . At higher temperatures the dominant mechanism is 2N ads → N 2. (c) N 2O and NH 3 are formed via N ads + NO ads → N 2 O, and N ads + 3 H ads → NH 3 the contributions of which respectively decre increase with increasing temperature, (d) The selectivities to N 2, NH 3 and N 2O are determined by the relative concentrations of NO ads, N ads and H ads which vary with the experimental conditions such as the temperature.