Performance, long-term stability and selectivity of fast potentiometric CO2-sensors constructed by combining a Na,CO, measuring electrode, a Na-beta "-alumina electrolyte and a reference electrode consisting of Na2Ti6O13/TiO2 or Na2Ti3O7/Na2Ti6O13 two phase mixtures are described. The materials of the reference electrode are chemically inert against CO2, so that the whole cell, without sealing the reference electrode, can be exposed to the ambient gas atmosphere. The sodium activity of the reference electrodes is fixed by the presence of the two phase mixture and the contact with oxygen in the gas phase. Thus thermodynamically defined, the sensor signal is essentially drift-free. Moreover, the influence of the oxygen partial pressure fixing the activity of the elements on both sides, cancels in the overall cell reaction. In this paper special emphasis is laid on demonstrating the fast Nernstian response with respect to CO2 and the independence of the signal from the oxygen partial pressure at temperatures above 500 degrees C.