Thin-film catalysts of gold and cobalt oxide were prepared by magnetron radio frequency sputtering deposition on magnesia substrate by cosputtering Au and Co3O4. We investigated the catalytic activity of this thin-film catalyst for CO gas and found that the catalytic activity was influenced strongly by the contents of the supported oxides in the thin film. Using this catalyst, a thermoelectric gas sensor of [Au, Co3O4]/SiGe/magnesia was developed to detect carbon monoxide gas. When this sensor is exposed to the gas mixture of air and CO, the catalytic reaction oxidation of CO heats up the catalyst-coated surface and then thermoelectric voltage builds up along the hot and cold region of the thermoelectric film of SiGe. The sensor of [Au, Co3O4]/SiGe/magnesia showed a temperature increase of about 6.6degreesC in the hot region and an output voltage signal of about 0.63 mV for 3% CO in air at 200degreesC. The sensor showed a clear voltage signal with good linearity for the target gas content in the range 0.05-3.0% and high selectivity for CO gas at 200degreesC. (C) 2003 The Electrochemical Society.