The physico-chemical characteristics and the reactivity of sub-monolayerV 2 O 5 -WO 3 2 deNO x catalysts is investigated in this work by EPR, FT-IR and reactivity tests under transient conditions. EPR indicates that tetravalent vanadium ions both in magnetically isolated form and in clustered, magnetically interacting form are present over the TiO 2 surface. The presence of tungsten oxide stabilizes the surface V I V and modifies the redox properties ofV 2 O 5 2 samples. Ammonia adsorbs on the catalysts surface in the form of molecularly coordinated species and of ammonium ions. Upon heating, activation of ammonia via an amide species is apparent. V 2 O 5 -WO 3 2 catalysts exhibits higher activity than the binary V 2 O 5 2 and WO 3 2 reference sample. This is related to both higher redox properties and higher surface acidity of the ternary catalysts. Results suggest that the catalyst redox properties control the reactivity of the samples at low temperatures whereas the surface acidity plays an important role in the adsorption and activation of ammonia at high temperatures.