The influence of residual chlorine species, originating from catalyst preparation procedures, on the carbon monoxide adsorption/desorption characteristics of Rh/CeO2 catalysts and on the oxidation state of the supported rhodium particles is studied by FTIR and X-ray photoelectron spectroscopy. Depending on the preparation procedure and the experimental conditions, three rhodium species may coexist on the catalyst surface, i.e. Rh-0, Rh+, and Rh3+. The presence of Cl species is found to stabilize an amount of rhodium in the Rh3+ oxidation state and to alter significantly the CO adsorption and desorption behavior of the catalysts. The kinetic rate of approach to chemisorptive equilibrium as well as the relative population and the thermal stability of the adsorbed CO species at equilibrium are significantly affected by the presence of Cl on the catalyst surface.