AbstractKinetic studies on the oxidation of carbon monoxide have been carried out between 413 and 473 K at different partial pressures of carbon monoxide and oxygen by means of the static method using vacuum‐activated, hydrogen‐reduced, and NiO‐doped In2O3semiconductors as catalysts. A strong carbon dioxide inhibiting effect is observed. The experimental data satisfactorily fit an equation derived by assuming the controlling step to be the adsorption of gaseous oxygen on the surface of catalyst. CO and CO2adsorb on the lattice oxygens (O o2−), while O2adsorbs on the oxygen vacancies (V ox) formed by vacuum‐activation, H2‐reduction, and NiO‐doping of In2O3. When CO2formed during the reaction is removed by means of liquid nitrogen trap, the oxidation is found to be first‐order with respect to CO and to be half‐order with respect to O2. The concentration of oxygen vacancy in the solid catalyst is shown to be the controlling factor for the oxidation of carbon monoxide. A possible reaction mechanism can be explained by then‐type character of In2O3and proposed from the experimentally obtained kinetic