The results of isothermal equilibration and X-ray diffraction studies in Fe2O3-rich compositions of the Na2Fe2O4-Fe2O3 pseudo-binary system suggests the coexistence of an equilibrium between alpha-Na(2)Pe(2)O(4) and Fe2O3 at low temperatures (923-1023 K) and Na2Fe2O4 and alpha-Fe2O3 at higher temperatures (>1023 K). The Gibbs energy of formation for the interoxide compounds Na2Fe2O4 and Na3Fe5O9 has been measured using solid-state electrochemical cells that use Na-beta-Al2O3 as the solid electrolyte. The electromotive force (emf) measurements are performed on cell I, which can be described as [GRAPHICS] and cell II, which can be described as [GRAPHICS] The temperature dependence of the steady-state emf of cells I and II can be represented as E-I(mV) = 258.3 + 0.177T (930-998 K) E-II(mV) = 399.6 + 0.040T (1025-1137 K) with a standard error of +/-5 mV for each emf measurement. By combining the measured emf values of cells I and II with the activity of Na2O in alpha-Al2O3 + Na-beta-Al2O3 from the literature, the computed Gibbs energy of formation of Na2Fe2O4 and Na3Fe5O9 from solid Na2O and alpha-Fe2O3 can be expressed as [GRAPHICS] where the standard error is +/-5500 J/mol for each Delta G degrees measurement. These uncertainty limits are due to the combined errors in the emf measurement and the thermodynamic data of the reference electrode material that have been used in the present study.