A solid state electrochemical cell, (-) CO2, O-2, Pt\0.92ZrO(2) . 0.08Yb(2)O(3) + Li2O . Sm2O3 . 2SiO(2)\Au, Li2CO3, CO2, O-2(+), was investigated for the monitor of carbon dioxide in air. A layer type ionic conductor prepared from Li+ ionic conductor and O2- ionic conductor by the thermal treatment was used as solid electrolytes. In the range of 450 to 600 degrees, the electromotive force, EMF, of the sensor increased linearly with an increase in the logarithm of CO2 partial pressure, in accordance with the Nernst lan;. On the other hand, the EMF was little affected by the change of O-2 partial pressure, At the detection electrode, the two-electron transfer and the four-electron transfer were found to occur for CO2 and O-2, respectively. The four-electron transfer reaction associated with O-2 molecules was found 40 occur at bath the detection and counter electrodes, indicating that this sensor has an excellent selectivity for CO2. The 90% response time was within 4 min. The EMF was not influenced by the introduction of water vapor.