The effect of vanadium content, temperature and calcination time on activity and selectivity of the biphasic Mg-3(VO4)(2)-MgO catalyst was studied. The oxidative dehydrogenation of ethylbenzene to styrene was used as a test reaction. X-ray diffraction (XRD), transmission and scanning electron microscopy (TEM and SEM) methods were applied to determine phase composition and microstructure of the catalysts. These results were compared with those obtained from the determination of specific surface areas, catalytic activity, selectivity and pore volume. The characterization of the active phase was performed by monitoring the styrene yield and detection of the total oxidation and cracking products. Non-homogeneous distribution of the vanadium in the active phase was found. These studies allowed us to obtain the highly-selective system for oxidative dehydrogenation of ethylbenzene. The mechanisms of this process and of by-products formation are shortly discussed.