Solid-state potentiometric CO2 sensors were fabricated using Na+ conductor (Na3Zr2Si2PO12) as a solid electrolyte, Li2CO3-BaCO3 fused carbonate with different compositions as a sensing electrode and the Na2Ti6O13-Na2Ti3O7 or TiO2-Na2Ti6O13 two phase systems as a reference electrode. The measured electromotive force of the investigated cells was found to be linear with the logarithm Of CO2 partial pressure in the range 100-12 600 ppm of CO2 and in the temperature range 300-500 degreesC. The response time was ca. 40-120 s. The sensors exhibited reproducible signal during short-time experiments (<24 h). However, the unstable behaviour was observed during long-time measurements (several weeks) in case of some sensors, and it was found to be sensor composition dependent. A sensing mechanism is proposed on the basis of the sensing characteristics obtained. The possible explanation of the observed long-time instability of investigated sensors is also given.