Carbon monoxide, a toxic gas, is produced from incomplete combustion of fuel, e. g., gasoline and natural gas, and the amperometric CO sensor is one of the commonly used sensors. One of the common electrodes for this type of sensors is catalytic Pt on a Nafion membrane. The Pt electrode modified with Sn was studied to improve the sensitivity of CO. In the present study, the sensitivity of CO on a Sn-modified Pt/Nafion assembly was 5.5 times larger than that on a Pt/Nafion assembly in the concentration range 0-400 ppm. With a Pt-W/Nafion/Pt-C assembly, the CO sensing current on a Sn-modified electrode was also about three times larger than that on a nonmodified electrode in the concentration range 0-100 ppm. (C) 2003 The Electrochemical Society.