Catalysts of Mo supported on MgO-gamma Al2O3 were studied in the oxidative dehydrogenation of propane to propene. The catalysts were active and very stable, but the dehydrogenation selectivity was reduced by the formation of carbon oxides. Characterizations by XRD, XPS, Raman spectroscopy, electron paramagnetic resonance, and BET surface measurements were performed. The catalyst preparation method led to large and stable magnesium molybdate particles on the surface. EPR and XPS measurements gave clues about the fact that the active centers for the reaction include Mo5+ ions. A scheme for the surface architecture is proposed.