Reduction properties of alumina-supported Rh catalysts together with a polycrystalline Rh foil were studied by a stepwise thermal decomposition in high vacuum. To investigate the progress of the oxide layer decomposition in detail, the samples were annealed in vacuum at various temperatures. After each annealing step, the changes in the chemical state of the noble metal were detected by XPS measurements. Our results show marked differences in the rate of decomposition between catalysts of different Rh loadings. In thermal decomposition, the formation of molecular oxygen and the subsequent desorption of O-2 decreases the oxygen content within the information depth of the XPS measurement. On the other hand, diffusion of oxygen from the deeper layers brings more oxygen to the selvedge. The observed decomposition properties are found to correlate with the particle size. In small particles the average diffusion length of oxygen atoms is shorter than in larger particles and the surface area is larger. These factors favor the decomposition of smaller particles in comparison with larger ones. (C) 2002 Elsevier Science B.V. All rights reserved.