A critical view is presented of the feasibility and reliability of electrochemical sensors combining Na-beta -alumina (i.e. Na2O . xAl(2)O(3)) solid electrolyte material with an in-situ created auxiliary phase and electrochemical sensors using a Na-beta -alumina based reference. Thermodynamic information on the stability of Na-beta -alumina and the activity of Na2O in the Na2O-xAl(2)O(3) binary system and in the Na-beta -alumina phase in particular, is critical in applications for sensors, and especially in situations where the operating temperatures are high and the oxygen pressures are low and where the Na-beta -alumina phase is used as a part of the electrode providing reference potentials in galvanic cells.