A series Of V2O5/Nb2O5 catalysts with vanadia loading varying from 2 to 12 wt.% were prepared and characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), temperature programmed reduction (TPR), BET surface area and oxygen chemisorption at 640 K. The catalytic properties have been evaluated for vapor phase ammoxidation of 3-picoline to nicotinonitrile. XRD results suggest the formation of beta-(Nb,V)(2)O-5 phase at higher loadings. TPR profiles showed two peaks: the low temperature peak is due to reduction of surface vanadia species and the high temperature peak is due to reduction of Nb2O5. XPS results reveal that both vanadia and niobia are present in fully oxidized state (5+) in all the samples. The intensity ratio V 2p(3/2):Nb 3d(5/2) is found to increase with increase in vanadia loading up to 6 wt.% and to remain constant at higher vanadia loadings. The oxygen uptake increases with increase of vanadia loading on niobia, whereas the dispersion of vanadia decreases. The dispersion of vanadia measured by oxygen chemisorption method is in good agreement with the dispersion determined from XPS. The ammoxidation activity 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 3-picoline are related to the oxygen chemisorption sites. (C) 2002 Elsevier Science B.V. All rights reserved.