Abstract: An electrochemical carbon monoxide detecting method and apparatus (10, 46) is disclosed which can detect carbon monoxide levels as low as about 500 ppm and which has excellent selectivity and a linear response over a broad range of carbon monoxide concentrations. The apparatus (10, 46) employs an electrical cell assembly (12, 54) having an electrolyte (20, 60) containing hydrated Cu(II) ions therein; upon creation of a constant magnitude potential difference between the cell electrodes (22, 24) ranging from about +0.03-+0.15 V, the Cu(II) ions are reduced to Cu(I) ions, and the latter react with carbon monoxide to form Cu(I)-carbonyl complexes. Detection of an electrical parameter indicative of the Cu(II)-Cu(I) reduction permits quantitative carbon monoxide determinations. Preferably, the detecting apparatus (14) is amperometric, employing a potentiostat (28) and output device (38). Alternately, detection can be accomplished potentiometrically.