Improvement in the efficiency of solid electrolytes requires the use of low electrolyte operating temperatures even though the ionic conductivity of electrolytes generally decreases at low temperatures. In this study, we propose the use of gadolinia-doped ceria (GDC) that has relatively high conductivity even at low temperature (200 degrees C) as a solid electrolyte for potential application in liquid sodium, and we investigate its thermochemical stability in the liquid sodium environment. The results of microstructural analyses, thermochemical stability tests, and total conductivity measurements in liquid sodium at 200 degrees C are discussed. Potentiometric cell with gadolinia-doped ceria (GDC) as a solid electrolyte and a In/In2O3 reference electrode were also tested in oxygen saturated liquid sodium environment in order to measure the emf. The emf in sodium environment is compared with theoretical data. The emf values measured in this study show a narrow range at low temperatures; however, the emf rapidly decreases with increasing temperature. The reason is caused by the mixed ionic electronic conductivity of GDC in oxygen-saturated sodium environment. (C) 2015 The Electrochemical Society. All rights reserved.