Kinetics of the re-oxidation of the H-2-reduced or vacuum treated VOx/TiO2 catalyst was studied. It was found that oxygen does not adsorb in the form of ad-atoms on the fully oxidized surface of the catalyst but re-oxidizes oxygen vacancies. On the basis of the surface potential measurements it was concluded that nucleophillic oxygen O-(S)(2-) is almost exclusively present above 550 K on the catalyst surface. The quantum-chemical calculations were supported by the experimental results. The implications of these results for the reaction mechanism of the oxidative dehydrogenation of propane are discussed. (C) 2007 Published by Elsevier B.V.