Isotopic tracers studies were performed to investigate the reaction network of selective catalytic reduction (SCR) on vanadia catalysts supported over titania. The catalysts were characterized using BET surface area analysis, X-ray diffraction, laser Raman spectroscopy, scanning electron microscopy, X-ray photoelectron spectroscopy, and thermal analysis techniques. The product analysis was carried out by a combination of chemiluminescence NO, analysis, gas chromatography-mass spectrometry, and chemical titration methods. The isotopic labeling experiments were performed under steady-state conditions by using N-15- and O-18-containing species. The results obtained in these experiments were combined with the nitric oxide and oxygen exchange experiments to arrive at conclusions regarding the pathways of the SCR reactions. The role of ammonia oxidation has been quantified in the overall SCR reaction network at higher temperatures.