The reactivity of ternary V2O5-WO3/TiO2 De-NOx catalysts is investigated by transient and steady-state techniques, and results have been compared with those obtained over binary V2O5/TiO2 samples having the same V2O5 loading. The results indicate that the reactivity of the ternary catalysts in the SCR reaction is higher than that of the vanadia-titania samples, and that at low temperatures the SCR reaction occurs via a redox mechanism that involves at first the participation of the catalyst lattice oxygen and then the reoxidation of the reduced sites by gas-phase oxygen. Accordingly, the higher reactivity of the ternary catalysts has been related to their superior redox properties.