A systematic variation of the V2O5 content of the title catalysts reveals a dependence of the activation energy of the selective catalytic reduction of nitric oxide by ammonia on the surface concentration of vanadia. A significant decrease in activation energy (from approximately 80 to 40 kJ/mol) is observed, as the vanadia surface concentration is increased (from 0.3 to 8 mu mol/m(2)). The same results were obtained from tests conducted both in the presence and absence of H2O and SO2. A possible mechanistic explanation is discussed.