In order to clarify whether MgMoxOy catalysts with slight excess of molybdenum relative to the stoichiometric MgMoO4 compound showed increased activities for propene formation in the propane oxidative dehydrogenation, we investigated the catalytic properties of MoO3 supported on MgMoO4 and of MgMoxOy catalysts treated with acid or base. Supporting MoO3 on magnesium-rich MgMo0.99Oy catalysts which are poorly active, or treating them with acetic acid to remove excess magnesium, resulted in drastic activity increases. On the other hand, the ammonia treatment of molybdenum-rich MgMo1.05Oy, catalysts which are highly active turned out to give a remarkable decrease in activity, because surface MoOx dissolved in ammonia water. A clear correlation was observed between the catalytic activities for propane oxidation and the dehydration of 2-propanol to propene over the supported catalyst and the treated catalysts. Since the bulk structures were unchanged by supporting or by the treatments, the existence of MoOx clusters formed on the surface of MgMoO4 are responsible for the activities in the oxidative dehydrogenation of propane.