Isotopic oxygen exchange on the surface of Mo-V-Te-Nb oxide catalysts was studied. Catalyst samples contained more than 95% of (i) orthorhombic phase M1, (ii) pseudo hexagonal phase M2, or (iii) layered phase with structure Me5O14 (Me - Mo, V, Nb). Two forms of exchangeable oxygen were found on the surface, i.e. strongly bonded lattice oxygen and relatively weakly bonded adsorbed oxygen. According to in situ studies with isotope O-18 adsorbed oxygen leads to deep oxidation products, while lattice oxygen participates mainly in the partial oxidation processes. Weakly bonded oxygen concentration decreases in series: Me5O14 > M2 > M1 and thus selectivity towards partial oxidation increases. (C) 2014 Elsevier B.V. All rights reserved.