Kinetic 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 Inz03 semiconductors as catalysts. A strong carbon dioxide inhibiting effect is observed. The experimental data satisfactorily fit an equatlon derived by assuming the controlling step to be the adsorption of gaseous oxygen on the surface of catalyst. CO and CO, adsorb on the lattice oxygens COZ-), while O2 adsorbs on the oxygen vacancies (V3 formed by vacuum-activation, H,-reduction, and NiO-doping of InzOs. When C02 formed 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 02. 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 the 11-type character of InzOj and proposed from the experimentally obtained kinetic data and conductivities.