Abstract: In a catalytic method for selectively detecting carbon monoxide in the presence of hydrogen in an oxygen-containing atmosphere, such as may be used in monitoring exhaust gases from combustion processes, the catalyst is a platinum-rhodium alloy containing not less than about 10% rhodium. The catalyst is maintained at a temperature such that it will cause combustion of carbon monoxide but will not readily cause combustion of hydrogen, the thermal effect on the catalyst resulting when a test atmosphere is brought into contact with it being sensed to provide a measure of the carbon monoxide concentration in that atmosphere.