Hydrogenation of nitric oxide on flat Ru(0 0 1) and stepped Ru(1 1 10) surfaces has been studied by Auger electron spectroscopy, X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy and thermal-desorption studies as well as by investigation of reaction kinetics. The products of the reaction were N2 and H2O under the investigated conditions (10–6–10–4 Pa of NO pressure and between 300 and 1100 K). A high reaction probability (0.5) was obtained on an oxygen-free clean surface, while the reaction did not take place on an oxygen-covered surface, which depends not only on the reaction temperature but also on the ratio of the partial pressures of NO to H2. Although the rate of formation of N2 on an oxygen-free clean surface is almost proportional to the partial pressure of NO above 520 K, the rate of desorption of N(ads) from the surface determines the overall rate of the reaction; N(ads) exhibited heterogeneous behaviour and its activation energy was shown to be 32 kcal mol–1. By comparison with the activity of N(ads) during ammonia decomposition, it was found that the rate of desorption of N(ads) in the reaction between NO and H2 was accelerated by the presence of a small amount of oxygen (perhaps < 1% of a monolayer). The activity of N(ads) above 520 K was the same for the flat and stepped surfaces, although the rate of N2 formation on the Ru(1 1 10) stepped surface was 1.5 times as fast as that on the Ru(0 0 1) flat surface at lower temperatures (450 K).