Periodic first-principles density functional (DFT) calculations have been performed to give an understanding of the active site and mechanism for the selective catalytic reduction (SCR) of NO by NH₃ over V₂O₅ in the presence of gaseous oxygen. Two synergetic functional groups, i.e. dual vanadium sites, have been found on the V₂O₅(010) surface: the hydroxy group (V–OH) containing vanadyl oxygen for the formation of NH₄⁺ species, and the VO group for the activation of the NH₄⁺ formed. The results have revealed that NH₃ is adsorbed preferentially on the Brönsted acid, and the NO hardly combines with the surface lattice oxygens to form NO₂, but rather gaseous NO interacts with the pre-adsorbed NH₄⁺ species to release N₂ and H₂O. This study suggests that the catalytic process of the SCR reaction follows an Eley–Rideal type mechanism.