An oxygen concentration cell of the type O-2(1), CO, Pt/stabilized zirconia, YSZ,/Pt, O-2(2) was found to be sensitive to a small amount of CO in an O-2-containing gas stream. Electrochemical measurements were carried out for the steady-state polarization curves in the temperature range of 500-650 degrees C. When CO coexisted in a stream, variation in polarization current was observed in the region within less than or equal to +/-50 mV of the equilibrium potential below 550 degrees C. In particular, the sensor showed good CO sensing characteristics in the concentration range of 32-800 ppm of CO in air. The CO sensing mechanism of the cell was also discussed.