An oxide capacitor consisting of BaTiO3and an oxide is studied as a new type CO2sensor based on capacitance change. Sensitivity to CO2, as well as the optimum operating temperature, was strongly dependent on the particular oxide mixed with BaTiO3. Among the elements investigated in this study, CuO–BaTiO3exhibited the highest sensitivity to CO2. In particular, the CuO–BaTiO3mixed oxide at the equimolar composition is highly sensitive to CO2. The optimum operating temperature and frequency for CuO–BaTiO3are 729 K and 100 Hz, respectively, and the 80% response time to 2% CO2is within 25 s. The equimolar mixture of CuO and BaTiO3can measure the CO2concentration from 100 to 60 000 ppm. Carbonation of oxide seems to play a key role for the detection of CO2on these mixed oxide capacitors. The optimum operating temperature of these mixed oxide capacitors for CO2detection, therefore, correlates with the decomposition temperature of the carbonate corresponding to the oxide mixed with BaTiO3. The capacitance increase of CuO–BaTiO3upon exposure to CO2seems to result from the elevated height of the potential barrier at the grain boundary between CuO and BaTiO3. Carbonation of CuO in the element seems to bring about the elevation in the height of the potential