Isotopic oxygen exchange experiments were carried out on simple oxides (Sb2O4, MoO3 and SnO2) and mechanical mixtures of MoO3 or SnO2 (acceptor phase) with Rh/Al2O3 or Sb2O4 (donor phase). With Rh/Al2O3 a synergy effect is found : the amount of oxygen exchanged on the mixture is higher than the sum of oxygen exchanged on the two separate phases. This is a prove of oxygen mobility between donor and acceptor phases. With Sb2O4 this effect is less marked suggesting that the presence of a reducer gas is required. In presence of hydrocarbon, no isotopic exchange is observed : only total oxidation occurs. The presence of Sb2O4 decreases the rate of total oxidation by blocking, by oxygen spillover, the most active sites for oxidation.