A series of MoO3/Nb2O5 catalysts with Mo loadings varying from 2.5 to 15 wt % were prepared and characterized by X-ray diffraction (XRD), temperature-programmed reduction (TPR), temperature-programmed desorption (TPD) of ammonia, electron spin resonance (ESR), oxygen chemisorption, and pore size distribution measurements. X-ray diffraction patterns indicate the presence of a crystalline molybdenum phase at higher Mo loadings on niobia. Dispersion of molybdenum was determined by the oxygen chemisorption at 623 K by a static method on the samples prereduced at the same temperature. At low Mo loadings, i.e., < 10.0%, molybdenum oxide is found to be present in a highly dispersed state. Pore size distribution studies indicate a decrease in average pore diameter and pore volume with increased Mo loading. ESR results suggest the presence of Mo5+ in the reduced catalysts. TPR results suggest that the reducibility of MoO3 increases with increased Mo loading. The reduction peaks due to niobia in TPR appeared at high temperatures (> 1173 K) and their intensity decreases with Mo loading. TPD of ammonia results suggest that acidity of the catalysts was found to increase with increased molybdenum loading. The catalytic properties were evaluated for the vapor-phase ammoxidation of 3-picoline to nicotinonitrile and related to oxygen chemisorption sites.