The CO sensing properties of Au/Co3O4, Au/SnO2–Co3O4, and Au/SnO2 catalysts deposited on micro thermoelectric gas sensor (micro-TGS) devices were investigated, and the catalytic properties of these catalysts were analysed by high-angle annular dark field scanning transmission electron microscopy (STEM-HAADF), STEM-energy dispersive X-ray spectroscopy (STEM-EDS), and diffuse reflectance infrared Fourier transform spectroscopy (DRIFT) evaluations. In all the catalysts, the Au particles were fully dispersed with an approximate grain size of 10nm. The CO sensing property of the micro-TGS containing the catalysts were found to depend on the catalytic supports of the catalysts. The observed decrease in the voltage signal of CO is a consequence of the reduction of the Sn/Co molar ratio of the catalyst. The results of the DRIFT analysis indicated that the interaction between CO and metallic Au is more pronounced for the Au/Co3O4 catalyst than for the Au/SnO2 catalyst. The voltage signal of micro-TGS with Au/Co3O4 catalyst was also higher than that with Au/SnO2 catalyst. This CO sensing property corresponded with the CO oxidation property, which was analysed by DRIFT, and these results indicated that the CO oxidation of Au nanoparticles was accelerated by the Co3O4 support. By controlling the Sn/Co ratio of catalytic supports, it becomes possible to control the CO–H2 response ratios of micro-TGS containing Au/SnO2–Co3O4 catalysts.