The role of each structural phase present in highly active and selective Ni0.85Nb0.15 catalyst in ethane oxidative dehydrogenation is investigated. Based on previous studies, Ni0.85Nb0.15 consists of a Ni-Nb solid solution and a Nb-rich amorphous phase. A series of Ni-based mixed oxides with low Nb content simulating the Ni-Nb solid solution and an amorphous Nb-rich sample were synthesized and studied separately. The catalytic tests demonstrated that the introduction of even a tiny amount of Nb (atomic ratio: Nb/(Ni + Nb) = 0.01) leads to a drastic increase in ethylene selectivity (150%) compared to pure NiO. Selectivity gradually increases with increasing Nb loading up to 15%, in parallel with the decrease in oxygen desorbed as measured by O-2-TPD. Based on the results of combined catalytic testing and physicochemical characterization, Ni-Nb solid solution can be identified as the key component for the high ethylene selectivity of the Ni0.85Nb0.15 catalyst, as the insertion of even a small amount of Nb drastically decreases the electrophilic oxygen species responsible for the total oxidation reactions. Co-existence of the Ni-Nb solid solution with the amorphous Nb-rich phase does not influence significantly the catalytic properties of the material. (c) 2011 Elsevier B.V. All rights reserved.