The three meta-, pyro-, and orthomagnesium vanadate phases have been synthesized, characterized, and tested as catalysts in the oxydehydrogenation of propane. In situ electrical conductivity measurements have clearly indicated that these solids behave as intrinsic semiconductors in oxygen at reaction temperature (520 degrees C), with band gap energies equal to twice the corresponding activation energies of conduction, in good agreement with the values determined by UV-visible spectroscopy. In the presence of propane, the electrical conductivity of the samples increased instantaneously at a variation rate and to an extent which both changed in the following order : pyro-Mg2V2O7 > meta-MgV2O6 > ortho-Mg3V2O8. The increase of conductivity has been attributed to the consumption of surface lattice oxygen anions by propane, leading to the formation of propene, water, and ionized anionic vacancies. The existence of these vacancies was supported by the study of their filling, using either dioxygen O-2 or nitric oxide NO, as sources of dissociated oxygen species. It is proposed that pure magnesium vanadates act as redox relays in the oxydehydrogenation of propane according to a Mars and van Krevelen mechanism. The higher catalytic activity of pyrovanadate Mg2V2O7 is directly connected with the lability of its surface O2- ions detected by conductivity. The high reoxidation rate implies that the solids are working in a surface state close to stoichiometry, i.e., close to the oxidized state. This could explain why the reaction kinetic order with respect to oxygen was equal to zero. The electrical properties of the three V-Mg-O phases could explain why pyrovanadate Mg2V2O7 was the most active and the most selective phase for oxidative dehydrogenation of propane into propene.