A solid-state CO2 sensor device using a metal-insulator-silicon carbide (MISiC) capacitor combined with a Li2CO3-BaCO3 auxiliary layer was fabricated and tested for its basic sensing properties. The MISiC-based CO2 sensor attached with the carbonate showed typical capacitance-voltage (C-V) properties in air at 400 degrees C, and the sensor device responded well to changes in CO2 concentration in air at 400 degrees C. The sensor signals were directly proportional to the logarithm of CO2 concentration. The results suggested that an electrochemical reaction of CO2 occurred at the interface between the carbonate layer and the electrode, causing the applied voltage to shift in response to CO2. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3512998] All rights reserved.