Quantum chemical calculations have been performed using density functional theory to model the mechanism of selective catalytic reduction of NO by NH3 on vanadium oxide. The reaction is initiated by NH3 adsorption on a Brønsted site modeled as a dimer cluster model representative of vanadium oxide, containing a terminal VO adjacent to a V−OH group. The calculations indicate that the adsorbed NH3 behaves as NH4+, which is supported by calculated IR spectra. Subsequently NO reacts with this activated NH3 to yield NH2NO and finally the reaction products N2 and H2O. The present results give support to a dual-site Eley−Rideal-type mechanism involving a Brønsted site and agree with isotopic labeling studies.