Potentiometric devices combining NASICON (Na+ conductor, Na3Zr2Si2PO12) with a series of Li2CO3-CaCO3 auxiliary phases were investigated for CO2 sensing properties as well as hetero-junction structure involved. It was revealed that an addition of proper amount of CaCO3 to Li2CO3 auxiliary phase was effective for improving CO2 sensing capability at lower temperatures; Li2CO3-CaCO3 (1:2) allowed normal operation down to 350 degreesC, while Li2CO3 did down to 400 degreesC. The analysis of fractured surfaces by means of an electron-probe micro-analyzer (EPMA) and SEM indicated the formation of an interfacial layer between NASICON and carbonates, the thickness and structure of which depended on the carbonate composition used. As revealed by X-ray diffraction analysis, CaZrO3 was formed between NASICON and Li2CO3-CaCO3 (1:2), suggesting its relevance to the lower temperature sensing capability.