Isotopic labeling studies of the reaction between (NO)-N-15 and (NH3)-N-14 have been performed, under selective catalytic reduction (SCR) conditions, for amorphous and crystalline chromia catalysts over a wide range of temperatures (140-350 degrees C) and oxygen concentrations (0-1.8% O-2. At low temperatures, and with 1.8% O-2, nitrogen is formed largely by the selective reduction of NO and NH3 over both catalysts. However, crystalline chromia has a much higher activity for ammonia oxidation. Thus, at approximate to 200 degrees C, the major form of nitrogen produced by amorphous chromia is (NN)-N-14-N-15, whereas, for crystalline alpha-Cr2O3, nitrogen is mainly N-14(2) and is therefore produced largely from ammonia oxidation. The dominant form of nitrous oxide produced in the presence of O-2 over both morphologies of chromia is always (NNO)-N-14-N-15. Thus, formation of N2O, an undesirable product, involves the reaction of one molecule of NO and one molecule of NH3. It has been shown that Fourier transform infrared (FTIR) spectroscopy can be used to distinguish between (NNO)-N-14-N-15 and (NNO)-N-15-N-14. In the presence of excess O-2, NO decomposition is unimportant for both catalysts. In the absence of O-2, very similar product distributions are observed for the two morphologies of chromia. The dominant form of nitrogen is (NN)-N-14-N-15, but the nitrous oxide is largely (N2O)-N-15, and therefore formed by NO decomposition. Evidence is presented for some conversion of N2O to N-2 in the absence of oxygen by a reaction conforming to 3N(2)O + 2NH(3) --> N-4(2) + 3H(2)O. This reaction increases in importance with temperature. Concentrations of O-2 comparable to the NO and NH3 concentrations (i.e., approximate to 1000 ppm) are sufficient to prevent the NO decomposition reactions, implying that O-2 effectively competes with NO for the available adsorption sites. Product distributions obtained in the presence of small amounts of O-2 are very similar to those achieved with excess O-2.