The transient behaviour caused by the change of the component concentration for CO oxidation on the perovskite-type catalyst La0.4Sr0.6Co0.4Mn0.6O3 was investigated. Results showed that CO was not adsorbed on the catalyst surface and CO oxidation was carried out between the surface oxygen species and gas phase CO. On the other hand, CO2 can be adsorbed on the catalyst surface but its adsorption site was different from the forming site.