An approach is described which, for the first time, enables the beta/beta "-equilibrium in Na-(beta + beta ")-Al2O3 to be characterized as a function of the phase composition. The study is based on a potentiometric solid state oxygen concentration cell. From the temperature dependence of the equilibrium constant, the thermodynamic stability of the beta "-phase is determined. The results lie in the middle of a fairly large scatter range spanned by the literature data. In view of the fact that in none of the previous investigations the establishment of the beta/beta "-equilibrium was in-situ checked, the present findings may be regarded as much more reliable than the respective data from literature. (C) 2000 Elsevier Science Ltd. All rights reserved.