The role was studied, of bulk V2O5 in vanadia/titania catalysts in partial oxidn. of toluene. Two catalysts with 1.8 and 11.1% of V were studied, by in situ DRIFTS and transient-response methods. Bulk V2O5 was found by FT-Raman spectroscopy only in the 11.1% V/TiO2 catalyst while monolayer vanadia species (monomeric and polymeric) were obsd. in both samples. Toluene interactions in the presence of gaseous oxygen showed that the selectivity to benzaldehyde and benzoic acid was similar for the two catalysts. The nature of adsorbed species was also similar. Toluene interaction with oxidized samples in the absence of gaseous oxygen led to rapid formation of coordinatively adsorbed benzaldehyde (.apprx.1625 cm-1) and benzoate/carboxylate species (1600-1400 cm-1), accompanied by the disappearance of monomeric vanadia species (.apprx.2037 cm-1). The catalyst contg. V2O5 showed higher level of toluene conversion to benzoic acid than the catalyst without V2O5. Large quantities of gaseous byproducts (COx, benzaldehyde, H2O) were formed when using the V2O5 contg. catalysts, while almost none were obsd. in the absence of V2O5. Thus, bulk V2O5 supplies oxygen to the vanadia active species; also, oxygen is involved in formation and desorption of intermediates and products. [on SciFinder (R)]