The oxidative dehydrogenation of ethylbenzene to styrene was studied on single-phase magnesium vanadates Mg2VO4, Mg-3(VO4)(2), Mg2V2O7 and MgV2O6. Both the V-V Mg-3(VO4)(2) and the V-IV Mg2VO4 were found to be active and selective, while MgV2O6 and Mg2V2O7 showed low styrene selectivities. The V-V Mg-3(VO4)(2) and the V-IV Mg2VO4 shared similarities in structure and underwent partial phase interconversion via reduction and oxidation. The V-V Mg-3(VO4)(2) was stable under the flow reaction conditions, giving a rather constant styrene selectivity of about 80% between 450 and 525 degrees C. However, the styrene selectivity was decreased by increasing the O-2/ethylbenzene ratio in the feed. Adding steam to the feed increased the styrene selectivity slightly. The V-IV Mg2VO4 was gradually oxidized under the dow reaction conditions. Also studied were the reducibilities of these vanadates, It was found that the reducibility of an oxide as measured by H-2-TPR could be very different from that deduced from its reaction with hydrocarbon. The nature of the lattice oxygen and its interaction with different reductants are discussed to explain the difference in reducibility as well as in catalytic characteristics between the four magnesium vanadates.