The reduction mechanism of nitric oxide by ethylene in the presence or absence of oxygen on mono- and multi-layer V2O5/ZrO2 and the structures of the catalysts under reaction conditions have been studied by Fourier transform infrared spectroscopy as well as by analysis of the reaction products. For the reaction of a mixture of NO+C2H4, only carboxylate species were observed at higher temperatures, although at lower temperatures nitrate species were formed. No bands due to a complex of NO+C2H4 were observed. From the results, it is proposed that ethylene is oxidized by the catalyst to form carbon dioxide via carbonyl and carboxylate species and nitric oxide reoxidizes the catalyst to form nitrogen. The quantitative analysis of the V=O band in the region of 1100-900 cm(-1) by band shape analysis indicates that only the surface V=O species in the top layer of the catalyst interacts with the adsorbed species.