This investigation reports the influence of MoO3 on the dispersion of vanadium oxide on zirconia support. Samples containing various MoO3 loadings ranging from 0.5 to 4 wt.% were prepared by impregnation of previously prepared 6 wt.% V2O5/ZrO2 with requisite amounts of ammonium heptamolybdate solution. Dispersion of vanadia was determined by oxygen chemisorption at 640 K and was found to decrease with the increase of molybdena loading. The calcined catalyst samples were characterized by specific BET surface area, XRD, FT-IR, Oxygen chernisorption and TPR of H-2 techniques. The XRD results show that the intensity of tetragonal phase decreases and monoclinic phase intensity increases with the addition of MoO3 to V2O5/ZrO2 catalyst. The TPR of H-2 results reveal that the reducibility of vanadia was found to decrease with the addition of molybdena to V2O5/ZrO2 catalyst. The catalytic properties were evaluated during the ammoxidation of 3-picoline to nicotinonitrile. The activity in ammoxidation reaction did not change much with the addition of molybdenum oxide. However, the selectivity of nicotinonitrile was found to increase with molybdena loading, indicates that the added molybdena created additional sites for the ammoxidation of 3-picoline reaction. The addition of molybdenum oxide inhibited the interaction between vanadium and zirconia, leading to decrease in dispersion and reducibility of vanadia. (c) 2006 Elsevier B.V. All rights reserved.