Among base metal oxides, Co₃O₄, MnO₂, and NiO, which have p-type semiconducting properties, hold potential catalytic capability for the oxidation of carbon monoxide (CO) at temperatures below 0^oC. Either of the following requirements should be fulfilled; low moisture content in reaction feed, optimum temperature and atmosphere for catalyst pretreatments, and the control of metal oxide morphology. Surface oxygen vacancies are assumed to play a key role in low temperature CO oxidation.