To improve the stability of Na3Zr2Si2PO12 (NASICON)-based potentiometric CO2 sensors fitted with carbonate/Au layers (sensing electrode), mixed conducting oxides of Li0.4CoO2 and Na0.6CoO2 were examined for their applicability to solid-reference electrodes in terms of their stability against CO2 and humidity. Compared with the Na0.6CoO2 electrode, the Li0.4CoO2 electrode showed far better stability against CO2. When the Li0.4CoO2 electrode was coated with a layer of glass, the electrode showed no response to CO2 (200-2000 ppm) and little interference from humidity [21-86% relative humidity (RH)] at 450 degrees C. A CO2 sensor device using Li2CO3-BaCO3 and a glass-coated Li0.4CoO2 showed stable electromotive force (emf) responses to CO2 in humid air (86% RH) at 450 degrees C. The emf of the device was also stable even after it was exposed to humid air (86% RH) containing CO2 (400 ppm) at room temperature for 1-2 days. The present study indicates the promising features of the glass-coated Li0.4CoO2 as a stable solid-reference material for NASICON-based CO2 sensors. (C) 2009 The Electrochemical Society. [DOI:10.1149/1.3216044] All rights reserved.