Mechanistic aspects involved in the formation of N-2 and of N2O during the reduction of gas-phase NO and of NOx stored over a model PtBa/Al2O3 NSR catalyst are investigated using unlabeled ammonia and labeled NO. The reduction of the stored NOx species (labeled nitrites and nitrates) with NH3 leads to the selective formation of N-2 as a major product and of small amounts of nitrous oxide. Based on the nitrogen isotopic distribution, it appears that N-2 formation occurs primarily through the statistical coupling of N-atoms formed by dissociation of NOx- and NH3-related surface intermediates, although an SCR-pathway (involving the coupling of NH3- and NO-derived ad-species) is also likely to occur. It appears as well that the formation of nitrous oxide involves either the coupling of two adsorbed NO molecules or the recombination of an adsorbed NO molecule with an adsorbed NHx fragment.