The effect of yttria addition to Na2CO3/BaCO3 composites for potentiometric CO2 sensors is studied in respect to ionic conductivity, microstructure, and sensing properties. The observed dependence of the sensing properties on both electrolyte and electrode materials is ascribed to varying contributions of yttria, Na2CO3, and BaCO3 phases to a mixed potential and to surface carbonate formation on yttria grains.