Activity studies and isotopic labelling experiments have been performed to determine the effect of water on the reaction of NO and NH3, in the absence and presence of oxygen, for both amorphous and crystalline (α-Cr2O3) chromia catalysts. The products of the reaction of 15NO and 14NH3 were determined using mass spectrometry, Fourier transform infrared spectroscopy, and gas chromatography. In the presence of excess (1.8%) O2, the addition of 1.5% H2O decreases NO and NH3 conversions over amorphous chromia for all temperatures below 275°C. For α-Cr2O3, the effect of water varies with reaction temperature: at temperatures less than 225°C, NO conversion is lower with water present, whereas above 225°C, higher NO conversions are observed under wet gas conditions. The effect of water on activity and selectivity is reversible. Labelling experiments show that water addition significantly decreases the formation of products arising from ammonia oxidation reactions. The formation of 14N2O and 14NO is completely inhibited for both forms of chromia. For α-Cr2O3, 14N2 is the dominant form of nitrogen under dry feed conditions. Increasing water vapour pressures suppress the formation of 14N2, and 14N15N becomes the dominant nitrogen species in the presence of 1.5% H2O. Increasing amounts of water favour the formation of 14N15N relative to that of 4N15NO for both amorphous chromia and α-Cr2O3. In the absence of O2, the activity and selectivity of both amorphous and α-Cr2O3 is markedly affected by the presence of small concentrations (0.5%) of added H2O, the effect being much more significant than in excess O2. Unlike reaction in the presence of oxygen, the selectivity to nitrous oxide increases with added water; however, the relative proportions of 14N15NO and 15N2O (the main nitrous oxide species) remain constant. A tentative mechanism, which can account for the observations, is given.