The nature of the vanadium species present on V2O5/Al2O3 catalysts was investigated by using solid state reflectance spectroscopy (DRS), X-ray diffraction (XRD) and temperature programmed reduction (TPR). V-51 NMR and DRS analyses indicated the presence of V5+ in tetrahedral symmetry at low vanadium loading. A surface polymeric vanadium species and/or the bulk crystalline V2O5 were mainly observed at high vanadium loading as also detected by XRD. The positions of the absorption edges determined through the UV-VIS spectra allowed distinguishing between various tetrahedral symmetries. After TPR, the average oxidation state of vanadium depended on the vanadium content. The nature of vanadium species was related to the catalyst behavior on the benzene oxidation reaction. The catalysts containing high vanadium content were more active suggesting that a high amount of V4+ is responsible for the higher activity. (C) 2001 Elsevier Science B.V. All rights reserved.