Ruthenium supported on magnesia was found to be a highly active and selective catalyst for the reduction of NO to Na with H-2. The adsorption of NO on Ru/MgO was studied at room temperature by applying frontal chromatography with a mixture of 2610 ppm NO in He. Subsequently, temperature-programmed desorption (TPD) and temperature-programmed surface reaction (TPSR) experiments in H-2 were performed. The adsorption of NO was observed to occur partly dissociatively as indicated by the formation of molecular nitrogen. The TPD spectrum exhibited a minor NO peak at 340 K indicating additional molecular adsorption of NO during the exposure to NO at roam temperature, and two N-2 peaks at 480 K and 625 K, respectively. The latter data are in good agreement with previous results with Ru(0001) single-crystal samples, where the interaction with NH3 was found to lead to two N-2 thermal desorption states with a maximum coverage of atomic nitrogen of about 0.38. Heating up the catalyst after saturation with NO at loom temperature in a H-2 atmosphere revealed the self-accelerated formation of NH3 after partial desorption of N-2, whereby sites for reaction with H-2 become available. As a consequence, the observed high selectivity towards N-2 under steady-state reduction conditions is ascribed to the presence of a saturated N + O coadsorbate layer resulting in an enhanced rate of N-2 desorption from this layer and a very low steady-state coverage of atomic hydrogen. The formation of H2O by reduction of adsorbed atomic oxygen is the slow step of the overall reaction which determines the minimum temperature required for full conversion of NO.