Vanadium oxide catalysts with V2O5 loadings ranging from 2.5 to 20 wt.% supported on Al2O3-ZrO2 (1:1 wt.%) mixed oxide have been prepared by wet impregnation method. The calcined samples were characterized by X-ray diffraction (XRD), BET surface area, pulse oxygen chemisorption, electron spin resonance (ESR), temperature-programmed reduction (TPR) of H-2, X-ray photoelectron spectroscopy (XPS), and UV-vis diffuse reflectance spectroscopy (UVDRS). The catalytic properties have been evaluated for vapor phase ammoxidation of toluene to benzonitrile. Dispersion of vanadia was determined by the pulse oxygen chemisorption method at 643 K. Vanadia is found to be present in a highly dispersed state at lower loadings and dispersion decreases steadily with increase of vanadia loading. The ESR spectra obtained under ambient conditions shows the presence of V4+ in tetrahedral symmetry. The TPR results show a single reduction peak corresponding to V5+-> V3+. XPS results reveal that vanadium is present in a fully oxidized state (+5) in all the samples. The intensity ratio V 2p(3/2)/Al 2P(3/2) and V 2p(3/2)/Zr 3d(5/2) is found to increase with increase in vanadia loading up to 12.5 wt.% and levels off at higher vanadia loadings. The UV-vis diffuse reflectance spectra indicate the existence of isolated and clusterized tetrahedral (Td) V5+ species in all catalysts. Ammoxidation activity increases with vanadia loading up to 12.5 wt.%, which corresponds to monolayer coverage and remains constant at higher vanadia loadings. The catalytic activity in ammoxidation of toluene to benzonitrile has been correlated to the oxygen chemisorption sites. (c) 2005 Elsevier B.V. All rights reserved.