A series of V2O5/AlPO4 catalysts with vanadia loadings ranging from 2.5 to 20 wt % was prepared and characterized by X-ray diffiraction (XRD), Fourier transform infrared spectroscopy, electron spin resonance spectroscopy, temperature-programmed reduction (TPR), temperature-programmed desorption (TPD) of NH3, BET surface area, and oxygen chemisorption methods. The catalytic properties were evaluated for the vapor phase ammoxidation of 3-picoline to nicotinonitrile. TPR results indicated that the appearance of a single reduction peak corresponds to V5+ to V3+. The XRD results suggest that, at low vanadia loadings AlPO4 is found to be amorphous. However, at V2O5 loading of 7.38 wt % and above, AlPO4 exist as alpha-cristobalite form in addition to V2O5. TPD results show that the acidity increases with vanadia loading and decreases at 7.38% and above loadings. The ESR spectra obtained under ambient conditions for the samples reduced at 640 K show the presence of V4+ in axial symmetry. Dispersion of vanadia was determined by oxygen chemisorption at 640 K and by the static method on the samples prereduced at the same temperature. The oxygen uptake measured at 640 K increases with increase in vanadia loading, whereas dispersion of vanadia. was decreased. The decrease in dispersion of vanadia was discussed in terms of formation of crystalline forms of V2O5 and AlPO4. The ammoxidation activity and selectivity of nicotinonitrile formation increase with vanadia loading up to 9.5 wt % and did not change appreciably at higher loadings. The catalytic properties during ammoxidation are related to the oxygen chemisorption sites.