The CO oxidation performance of the Au/Co3O4 catalyst on the micro gas sensor device with different treatment conditions has been investigated. The CO oxidation performance of the Au/Co3O4 catalyst on the micro device has been improved by the H-2 combustion treatment under the flow of H-2 in air and the reduction treatment under the flow of H-2 in N-2. The H-2 combustion treatment of the Au/Co3O4 catalyst on the micro device prevented the degradation of the CO oxidation performance. To investigate the chemical states and microstructures of Au and Co in the Au/Co3O4 catalysts after the reduction and H-2 combustion treatments, transmission X-ray absorption fine structure (XAFS) measurement and Xray photoelectron spectroscopy (XPS) have been performed. Au particles of Au/Co3O4 were found to be metallic Au regardless of the treatment conditions. However, a hydroxyl group bonded to the transition metal Co as a form of Co(OH)(2) was found on the catalyst surface after the H-2 combustion treatment. The H-2 combustion treatment of Au/Co3O4 results in the formation of OH-in the metal-support interface, which is considered to be an origin of the improvement of the CO oxidation performance of the Au/Co3O4 catalyst. (C) 2012 Elsevier B. V. All rights reserved.