Tishchenko reaction of benzaldehyde was carried out over the MgO whose surface O atoms were enriched with O-18 to examine whether the surface O atoms of MgO are incorporated into the product and reactant. The surface lattice O-18 atoms were incorporated in both the product and reactant. The O-18 concentration was significantly higher for the product than for the reactant. In addition, the O-18 in the product ester was located not only in the carbonyl group (C=O) but also in the bridging position (C-O-C); O-18 exchange between benzaldehyde and MgO surface occurred not only in the adsorption-desorption of benzaldehyde, but also in the dimerization process. The oxygen exchange results in the migration of active sites of coordinative unsaturated site (CUS) over the surface of MgO catalyst during the reaction. (C) 2006 Elsevier B.V. All rights reserved.