Synergetic effects in the oxidative dehydrogenation of propane have been studied over magnesium vanadate catalysts containing three different Mg/V ratios : 1/2, 2/2, and 3/2, denoted as MgV(1/2), MgV(2/2), and MgV(3/2). Four types of catalysts were analysed : (a) pure magnesium vanadate oxides, (b) mechanical mixtures of the pure magnesium vanadate oxides, (c) mechanical mixtures of the magnesium vanadate oxides with alpha-Sb2O4, and (d) impregnated MgV(2/2) and MgV(3/2) with Sb ions. Synergetic effects are observed in MgV(3/2) and in MgV(2/2) oxides when they are in presence of (alpha-Sb2O4. In the mixtures of MgV(3/2) with alpha-Sb2O4, the principal effect is an increase in the selectivity with a corresponding decrease in propane conversion, whereas in the mixtures of MgV(2/2) with alpha-Sb2O4, there is a strong increase in propane conversion with a moderate increase in propene yield. Concerning the mixtures of MgV(3/2) and MgV(2/2), synergetic effects in the conversion, in the yield, and in the selectivity are observed. However, no synergetic effects in selectivity or conversion are exhibited by MgV(2/2) and MgV(3/2) when they are mixed with MgV(1/2). Highly dispersed (SbxOy), formed on impregnated MgV(2/2) and MgV(3/2), sinters and detaches from MgV(2/2) and MgV(3/2) surfaces. No formation of a new phase or contamination in the presence of alpha-Sb2O4 takes place when MgV(3/2) is used. Thus, these catalysts contain two separate phases in contact. In MgV(2/2) + alpha-Sb2O4 mechanical mixtures a new phase, MgSb2O6, (formed during the test) is also present in small quantities. The mixtures of MgV(3/2) with MgV(2/2) reveal neither formation of a new phase nor contamination. The synergetic effects in the selectivity exhibited by MgV(3/2) mixed with MgV(2/2) and with alpha-Sb2O4 is explained by a remote control mechanism. MgSb2O6, formed in MgV(2/2)-containing catalysts, is responsible for the increase in complete oxidation and the strong decrease in selectivity.