A solid-state planar electrochemical carbon monoxide sensor was built and studied. The sensor features a planar geometry of the electrodes (working, counter, reference), where all three electrodes are located on the same side of a substrate. The electrodes are covered by the CO-semipermeable proton-conducting polymer film. The sensor shows characteristics of a practical-use level: response time, 30 s; range, 0-2000 ppm; sensitivity, 0.04 nA/ppm; life, more than 2 yr. The sensitivity is governed by both the CO permeability of the polymer film and the CO oxidation rate, in other words, by the intermediate kinetic mechanism.