Isotopic tracer studies were performed to investigate the reaction network of selective catalytic reduction of nitric oxide over vanadia catalysts having preferential exposure of different crystal planes. The catalysts were characterized using BET surface area analysis, X-ray diffraction, laser Raman spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, 3-D imaging, and thermal analysis techniques. The product analysis was carried out by a combination of chemiluminescence NOx analysis, gas chromatography-mass spectrometry, and chemical titration methods. The isotopic labeling experiments were performed under steady-state reaction conditions by using NH3 + NO + O-16(2) --> NH3 + NO + O-18(2), (NH3)-N-14 + NO + O-2 --> (NH3)-N-15 + NO + O-2, NH3 + (NO)-N-14 + O-2 --> NH3 + (NO)-N-15 + O-2, and NH3 + (NO)-N-14-O-16 + O-16(2) --> NH3 + (NO)-N-15-O-18 + O-18(2) switches. Interaction of NO with the vanadia surface was also studied using a (NO)-N-14-O-16 --> (NO)-N-15-O-18 switch. The results obtained in these experiments were combined with the tracer studies performed in ammonia oxidation reactions over the same catalysts to elucidate the reaction pathways involved in SCR reactions, to compare the surface residence times of nitrogen containing species, as well as to quantify the role of ammonia oxidation in SCR reactions.