A study of CO oxidation on LaCoO3 perovskite was performed in an ultrahigh vacuum system by means of adsorption and desorption. All gases were adsorbed at ambient temperature. Two adsorption states (α- and β-) of CO exist. The α-peak at 440 K is attributed to carbonyl species adsorbed on Co3+ ions while the β-peak at 663 K likely comes from bidentate carbonate formed by adsorption on lattice oxygens. CO2 shows a single desorption peak (β-state, 483 K) whose chemical state may be monodentate carbonate. A new CO2 desorption peak at 590 K can be created by oxidation of CO. O2 also shows two adsorption states. One desorbs at 600 K, which may reflect adsorption on Co3- ions. The other apparently incorporates with bulk LaCoO3 and desorbs above 1000 K. The two adsorption states of CO are oxidized via different mechanisms. The rate determining step in oxidation of a-CO is the surface reaction whereas for that of β-CO, it is desorption of product CO2.