A series of catalysts built up by V2O5 and WO3 overlayers on TiO2 anatase which have shown remarkable activity and selectivity in the selective catalytic reduction (SCR) of nitric oxide have been investigated by electron paramagnetic resonance spectroscopy. All the systems exhibit spectra due to the presence of both isolated and magnetically interacting V(IV) ions whose amount has been monitored as a function of the temperature of progressively reducing vacuum treatments. By comparison with binary systems (V2O5/TiO2 and WO3/TiO2) evidence has been provided for the structural features of V(IV) ions isolated at the surface of the ternary systems. Although the two supported phases mainly interact with the TiO2 matrix, the present investigation reveals the existence of a mutual (structural and electronic) interaction between the two supported bidimensional phases which determines the special redox properties of the ternary systems. These properties are remarkably different from that of V2O5/TiO2 systems and can contribute to an understanding of the catalytic behaviour of the V2O5-WO3/TiO2 systems in the SCR reaction.