Partial oxidn. of toluene on V/Ti oxide catalysts is a reaction of industrial interest. In spite of the detailed study of the nature of surface vanadia species, there is an evident lack of the information concerning their reactivity. The present work demonstrates an application of the transient response techniques in combination with \"in situ\" DRIFT and FT-Raman spectroscopy for the discrimination of the reactivity of different vanadia species. Catalysts consisting of 0.35-3.7 monolayers of vanadia on titania were studied. In anaerobic conditions, conversion of toluene to benzaldehyde (BA) with selectivity close to 100% was found. BA is formed mainly by nucleophilic (O2-) oxygen in isolated monomeric vanadia species, while electrophilic oxygen (O-) present in polymeric and bulk vanadia species is responsible for the formation of carbon oxides. An addn. of potassium was obsd. to increase the yield of BA. [on SciFinder (R)]