Study of oxygen exchange between vanadia-titania catalyst, H2O and O-2 has been performed in isothermal (200 degrees C) and thermo programmed (50-500 degrees C) conditions. Experiments using (H2O)-O-18, 18 02 and monolayer vanadia-titania catalyst placed to the plug-flow reactor were carried out. Significant values of the rate of isotope exchange between O-2 and the catalyst were observed at T>450 degrees C, but catalyst reduction accompanied oxygen desorption into gas phase proceeded in this case. Unlike dioxygen, water can exchange quite readily its oxygen atoms with vanadia-titania catalyst even at room temperature. Surface vanadium sites coordinating the OH group show the highest activity in the oxygen exchange with water. The rate constant of oxygen exchange between adsorbed water molecule and V-OH is ca. 0.5 s(-1) at 200 degrees C, with the activation energy close to zero. The rate of oxygen exchange with dehydrated vanadium complexes V-O-V and V=O is much lower. The rate constant of this exchange is ca. 10(-3) s(-1) at 200 degrees C, with the activation energy of ca. 70 kJ/mol. The interaction of adsorbed water with VOx species results in their slow hydrolysis to form the V-OH groups (characteristic time ca. 10(4) S), which recombination underlies the isotope exchange. (C) 2009 Elsevier B.V. All rights reserved