Within Mo5O14-like structural type for mixed V-Mo-Nb oxides, the effect of vanadium and niobium content and treatment temperature on the phase composition, structure and catalytic properties of the samples in oxidative dehydrogenation of ethane has been studied. The limits of V and Nb content in the ternary Mo5O14-like oxide are determined. V-Mo-Nb oxides are shown to be the substitution solid solutions of composition (Mo0.6VzNb0.4-z)(5)O-14 (0 < z < 0.2) based on binary oxide (Mo0.6Nb0.4)(5)O-14. The structure of crystalline V-Mo-Nb oxides is refined. Crystallization of Mo5O14-like structure occurs at temperatures >= 550 degrees C. Below 550 degrees C, V-Mo-Nb oxides are nanocrystalline materials having a disordered layered structure with the interlayer distance of ca. 4 angstrom. The structural model of nanocrystalline oxides is discussed. The role of Nb in the formation of ternary V-Mo-Nb oxide compounds is revealed, and the surface state of these oxides at different temperatures is analyzed. The low-temperature V-Mo-Nb oxides are the most active catalysts in the reaction of ethane oxidative dehydrogenation. They are characterized by the ratio V/Nb approximate to 1 in the Mo5O14-like structure. (C) 2009 Elsevier B.V. All rights reserved.