The effects of H2O and the vanadia content of the catalyst on the formation of15NH3 during the reaction of15NO and14NH3 in the absence of O2 over V2O5-based catalysts have been determined by mass spectrometry and Fourier transform infrared spectroscopy. At 450°C, the contribution of15NH3 to the total nitrogen-containing products remains constant at about 20% for water concentrations from 0 to 1.6%. The vanadia content also has little effect on the proportion of15NH3 produced. Combination reactions producing14N15N and14N15NO consume surface oxygen species and oxygen mass balances indicate that the amount of15NH3 formed is determined by the extent of these combination reactions. Small concentrations of O2 (<300 ppm) were sufficient to prevent the formation of15NH3. The reduction of NO by H2 was also studied. Negligible amounts of NH3 were formed under dry feed conditions, whereas, in the presence of 1.6% H2O,15NH3 represents about one third of the products. A mechanism involving reaction of an adsorbed Ns species with H2O is used to account for these experimental observations.