The study of oxidative dehydrogenation of propane was carried out over V-Mg-0 catalysts. The citrate method was used to prepare catalysts with the Mg/V atomic ratio varying from 1/4 to 9/1. Three pure Mg vanadates and the corresponding biphasic catalysts were obtained from very homogeneous Mg-V-citrate precursors after calcination at 550-degrees-C. Among the three pure Mg vanadates, alpha-Mg2V2O7 (pyrovanadate) was found to be the most selective, while MgV2O6 (metavanadate) is the worst one. Interestingly, the catalytic behavior of biphasic catalysts is quite different from that of pure Mg vanadate phases. For example, the selectivity of Mg3V2O8 (orthovanadate) can be promoted by a coexisting pyrovanadate phase or excess magnesium oxide in intimate contact. These differences are discussed in terms of the different specific activities of phases and possible synergy effects between the phases.