NASICON (Na3Zr2Si2PO12)-based electrochemical devices, constructed in a gas-separation type or planar type, were attached with a Li2CO3-BaCO3 (1:2 in molar ratio) auxiliary phase to investigate their CO2 sensing properties under ambient conditions. The devices were found to be far more resistant to the disturbance by a change in relative humidity ( RH) than those using Na2CO3 or NaHCO3 auxiliary phase. The EMF was correlated linearly with the logarithm of CO2 concentration in the range of 0.03-50% CO2 at 30degreesC, and the linear correlation shifted up or down only slightly with a change in RH between 30 and 90%. For a change in operating temperature, the CO2 sensing capability was kept almost the same at 45degreesC, while it was deteriorated significantly at 60 and 75degreesC, especially in the lower RH range. Applicability of a cobalt oxide bronze (Na0.6CoO2) as a solid reference electrode for the planar type device was also investigated. The devices using the bronze reference electrode exhibited the CO2 sensing capability as good as those using the Au reference electrode did. In addition, the bronze electrode was found to much improve the reproducibility of the EMF to a fixed concentration of CO2 among devices. (C) 2003 Kluwer Academic Publishers.