Successive O-2/H-2/O-2 chemisorption measurements were performed at room temperature over Pt/CeO2 catalysts of different BET surface areas after reduction at 573 K under H-2. It has been determined that about 4 mu atom of oxygen per m(2) of ceria is required to fill the oxygen vacancies created by the reduction in agreement with the value deduced from the TPR profiles. The same value was also obtained from the H-2/O-2 titration, which shows that oxygen is able to titrate the hydrogen which has spilled over the ceria surface via the metallic particles, Thus, the consistency of the three techniques demonstrates the validity of this method of measuring both the total ceria surface area and the ceria surface in contact with the precious metals. The application of this methodology to the case of Pt and Pt-Rh /CeO2-Al2O3 model three-way catalysts leads to a set of ceria surface areas deduced from O-2 chemisorption and O-2 titration much lower than those obtained by TPR. This behaviour has been attributed to the presence of chlorine ions originating from the precursor and to the formation of stabilised Ce3+ ions at the CeO2/Al2O3 interface during reduction, After an ageing treatment at 1273 K under N-2 + 10% H2O, an even greater difference was observed between the TPR ceria surface areas and those deduced from the chemisorption method. The results indicate an increased fraction of ceria without metal due to sintering and to the segregation between ceria and the metal particles. (C) 2000 Academic Press.