A thin film solid state electrochemical gas sensor has been investigated for CO2 detection based on the cell reaction Na+ + OH- + CO2 = NaHCO3. The galvanic cell arrangement is Au/NaxCoO2-delta (ref)/NASICON/Au, SnO2 with the right-hand electrode being exposed to CO2 and O-2. Polished NASICON pellets of 300-500 mu m thickness have been employed as well as electrolytes and substrates. The NaxCoO2-delta reference material, Au leads and a catalytic SnO2 film were deposited by rf-sputtering. For electrical insulation and encapsulation, the reference side of the sensor has been covered by a thin film of SiO2-xNy. On top of this thin insulating layer a thin Pt film and an integrated Pt-Pt/Rh thermocouple have been deposited also by rf-sputtering for heating the device and for temperature measurement, respectively.