A solid electrolyte type carbon dioxide gas sensor operable at moderate temperature was fabricated by using an Al3+ cation conducting (Al0.2Zr0.8)(20/19)Nb(PO4)(3) solid. The Nd2O2CO3-(0.52Li(2)CO(3)-0.48Na(2)CO(3)) solid and an Al metal thin film were selected as the auxiliary sensing electrode and reference electrode, respectively, to produce a sensor that exhibits superior sensing performance in terms of a quantitative, continuous, and reproducible response while obeying the theoretical Nernst relationship at temperature as low as 350 degrees C. The sensor exhibited stable CO2 gas detection over 2 months; therefore, it is expected to be applied as a practical on-site CO2 gas detection tool. (C) 2011 The Electrochemical Society. [DOI: 10.1149/2.009112esl] All rights reserved.