The kinetics of CO oxidation over two catalytic sites, either 100% zero-valent or 100% oxidised rhodium atoms, were studied. It was shown that an oxidised rhodium species exhibits a significant higher turnover rate in CO oxidation (factor 25) than the corresponding zero-valent one. This noticeable difference, interpreted with respect to mechanistic considerations, led us to conclude that one of the promoting effects of ceria-zirconia is to stabilise rhodium atoms as oxidised species.