The chemisorption of CO and hydrogenation of surface adsorbed carbon species formed after reaction of CO/He with Rh/MgO catalyst in the range 300-573 K were studied by transient methods employing both FTIR and mass spectroscopy in separate flow and reactor systems. Exposure of Rh/MgO to CO/He at 523 K results in a rapid formation of linear and bridged CO species. It was found that formation of gem-dicarbonyl CO species during CO chemisorption at 300 K is strongly affected by CO/He treatment at 523 K and H-2 reduction conditions. Experiments with labelled CO ((CO)-C-13 and (CO)-O-18) indicate that linear and bridged CO species readily exchange with gaseous CO species. Transient hydrogenation results for adsorbed carbon-containing species formed after CO/He reaction at 523 K provide evidence that H-2 chemisorption decreases after cycles of reaction in CO/He at 523 K followed by H2 reduction at 673 K. However, the surface coverages of adsorbed CO species are not affected by such treatments. Oxygen pretreatment of the catalyst at 623 K followed by H-2 reduction, after CO/He reaction at 523 K, greatly affects the transient kinetics of hydrogenation of adsorbed CO species at 523 K but not their respective surface coverages. This result is related to a change in the hydrogen chemisorption sites by oxygen treatment which resulted in the removal of inactive carbon not hydrogenated at 723 K. A kinetic model for the hydrogenation of the various adsorbed carbon-containing species is proposed which accounts for the interpretation of the CH4 transient responses obtained.