The relation between tungsten and vanadium oxide species in V2O5-WO3/TiO2 catalysts for the selective catalytic reduction of nitrogen oxides by ammonia was studied with two series of catalysts containing 0.5-5 wt.% V2O5 and 0 or 10 wt.% WO3 impregnated onto a high surface-area titania hydrate of anatase structure to keep theoretical surface coverages of the transition metal oxides <= 1. The catalysts, which were investigated by XRD, nitrogen physisorption, Raman and EPR spectroscopy, temperature-programmed reduction (TPR), and catalytic reduction studies, all exhibited the well-known acceleration of the SCR reaction by the tungsten promoter. From the EPR spectra and the TPR profiles, it was concluded that the presence of tungsten induces the formation of a surface oxide phase with intimate mixing between vanadium and tungsten oxide species instead of pronounced surface vanadium oxide island formation, which was found in the absence of tungsten. This suggests that the promotional effect of tungsten may originate from a direct influence on the neighboring vanadium oxide species or from the disruption of too large surface vanadium oxide ensembles. (C) 2011 Elsevier Inc. All rights reserved.