Two series of metal catalysts (Rh, Pt, Pd, Ru and Ir) supported over CeO2 and Ce0.63Zr0.36O2 were prepared. Catalysts were pretreated at 500 degrees C (fresh) and further sintered either in H-2 or in air at 700-900 degrees C. All catalysts were characterized by H2 chemisorption at -85 degrees C and/or by transmission electron microscopy (TEM). The O-16(2)+O-18(2) homoexchange reaction was carried out in the 200-500 degrees C temperature range. Rh, Ru and Ir showed the highest homoexchange rate R (per M metal) while Pd and to a lower extent Pt were less active. Sintering affected the metal performances differently: while R is higher on small Rh particles (fresh catalysts), the reverse situation could be observed for Ru and Ir where the sintered catalysts were more active than the fresh ones, especially in the case of CeO2. (c) 2005 Elsevier B.V. All rights reserved.