Heterogeneous catalysts that can work at ambient temperature are useful for in-door air quality control, pure gas production for fuel cells and semiconductors, gas sensing, and so forth. Deposition of gold nanoparticles on base metal oxide is known to provide highly active catalysts for CO oxidation at temperatures below room temperature. Here we report that some select base metal oxides such as Co3O4, MnO2, and NiO are intrinsically active catalysts for CO oxidation below 50 degrees C when pretreated at moderate temperature between 150 and 250 degrees C in a stream of non-reducing dry gases, for example, dry air, CO in air, or N-2. These metal oxides are p-type semiconductors and form surface excess oxygen adsorbed at oxygen vacancies which are created by the above pretreatments. (C) 2009 Elsevier Inc. All rights reserved.