Density functional theory (DFT) calculations were carried out on a vanadium oxide cluster containing four vanadium atoms to probe the mechanism of the selective catalytic reduction (SCR) of NO with ammonia. The interaction of ammonia with Bronsted acid sites on this V-4-cluster leads to the formation of NH4 species bonded to two vanadyl (V=O) groups, with a bonding energy of -110 kJ/mol. This adsorbed NH4 species reacts with NO in a series of steps to form an adsorbed NH2NO species, which subsequently undergoes decomposition to form N-2, H2O, and a reduced vanadium oxide cluster (V-4-H). The latter reaction occurs via a series of hydrogen-transfer steps by a "push-pull" mechanism with adjacent V=O and V-OH groups on the vanadium oxide cluster. The rate limiting process in this conversion of NO and NH3 to give N-2, H2O, and V-4-H involves the reaction of an adsorbed NH3NHO adduct to form NH2NO species. The transition state of this step may be stabilized through hydrogen bonding with surrounding vanadia and/or titania moieties.