A series of V2O5/Nb2O5 catalysts with V2O5 loadings ranging from 2 to 12 wt.% of V2O5 were prepared by wet impregnation method. These catalyst samples were characterized by X-ray diffraction (XRD), oxygen chemisorption, electron spin resonance (EPR), temperature-programmed desorption (TPD) of ammonia, and scanning electron microscopy (SEM). The catalytic properties of these catalysts were tested for vapor phase ammoxidation of toluene to benzonitrile and compared with more traditional supports, such as Al2O3, SiO2 and TiO2. Dispersion of vanadia was determined by the static oxygen chemisorption method at 640 K on the samples prereduced at the same temperature. At low vanadia loadings (<6 wt.%) vanadium oxide is found to be present in a highly dispersed state. The X-ray diffraction results suggest the formation of beta-(Nb,V)(2)O-5 phase at higher loadings of V2O5. The EPR spectra obtained under ambient conditions for the catalysts show the presence of V4+ ions in a distorted tetrahedral symmetry. SEM results suggest that at low vanadia loadings vanadia is well dispersed and at higher loadings dense clusters are formed. The results of temperature programmed desorption of ammonia indicate that the acidity increases with vanadia loading up to 6wt.% V2O5 and decreases at higher vanadia loadings. The ammoxidation activity also increases with vanadia loading up to 6 wt.%, which corresponds to monolayer coverage and remains constant at higher vanadia loadings. The catalytic properties during ammoxidation of toluene are correlated to the oxygen chemisorption sites and acidic sites. (C) 2004 Elsevier B.V. All rights reserved.