Drastic activity increases were observed by the treatments of the magnesium-rich MgMo0.99Oy catalysts, which are poorly active for the oxidative dehydrogenation of propane, with inorganic or organic acid to remove excess magnesium on the surface. MoO3 loading on magnesium-rich MgM0.99Oy catalysts also resulted in drastic activity increases. The activity increases followed non-effective loadings of MoO3 in the range 0-2 wt%, because it is necessary to neutralize the surface magnesium with MoO3 before the formation of molybdenum-rich surface. The pH of the aqueous (NH4)(6)Mo7O24 solution for the MoO3 loading apparently influenced the activity. Under the acidic conditions the MoO3 loading resulted in the drastic activity increase but under the basic conditions the effect of the MoO3 loading was poor, suggesting that a cluster-type MoO3 on MgMoO4 surface is responsible for the activity of propane oxidative dehydrogenation.