Potentiometric electrochemical sensor which can measure the oxygen content in the medium at temperatures lower than 600 degrees C, where zirconia sensor does not work has been developed adopting cation conducting electrolyte of Na beta-beta" alumina. The new structure of the oxygen sensor as shown below has a beta-beta" alumina as an electrolyte on both sides of which thin layer of yttria stabilized zirconia (YSZ) is attached. The YSZ layers are thought to maintain their oxygen activities in equilibrium with ambient oxygen partial pressure. O-2(RE), CuO, Cu2O vertical bar YSZ, Pt vertical bar Na beta-beta" alumina vertical bar Pt, YSZ vertical bar O-2(WE) The electromotive forces (EMF) measured from the above cell show well-defined logarithmic relation with oxygen partial pressure as low as 500 degrees C. However, it exhibited a lower sensitivity (about 10%) than what it should be for the theoretical value. It can be explained in terms of the electronic conduction occurring at Na beta-beta" alumina. (c) 2004 Published by Elsevier B.V.