Isotopic labeling studies of the reaction between (NO)-N-15 and (NH3)-N-14 have been performed over a range of vanadia-titania-based SCR catalysts (pure V2O5 and catalysts containing 1.4-23.2 wt % V2O5) for the extended temperature range of 200-500 degrees C. For temperatures less than 350 degrees C, (N15N)-N-14 is always the major product. At higher temperatures, however, product distributions are very sensitive to vanadia content; ammonia oxidation to (NO)-N-14 is particularly dominant for pure V2O5 and, at 500 degrees C, accounts for more than 70% of the nitrogen-containing products. Pure V2O5 also produces significantly more (NNO)-N-14-N-15, and at much lower temperatures, than that observed for a 1.4 wt % V2O5/TiO2 catalyst. On the basis of these results it is clear that ammonia oxidation to (NO)-N-14 is the major reason for the observed decrease in the NO conversion over vanadia-based catalysts at temperatures greater than 400 degrees C. Ammonia oxidation to nitrogen and nitrous oxide is less significant; (14)N2O and N-14(2) each comprise less than 10% of the total products for both the pure and supported vanadia catalysts. Addition of 1.6% water decreases the amount of nitrous oxide (largely (NNO)-N-14-N-15) produced over the supported catalyst at 450 degrees C by over 90%. A simultaneous increase in the amount of (NN)-N-14-N-15 is, also observed. The presence of water also suppresses the (NH3)-N-14 oxidation to N-14(2), (N2O)-N-14, and (NO)-N-14, even at 500 degrees C. By contrast, for pure V2O5 at 500 degrees C, water has a relatively minor effect on the product distribution, and the major product remains (NO)-N-14. In general, high temperatures, dry feed gas conditions; and high vanadia contents favor both the production of (NNO)-N-14-N-15 relative to (NN)-N-14-N-15,,and the ammonia oxidation reaction producing (NO)-N-14. Results from this and previous studies suggest that there is-a relationship between N2O formation and NH3 oxidation capability.