The standard Gibbs free energy of formation of Na2ZrO3 has been re-determined by a solid state electrochemical approach that allows the measurement to be performed without any distortion by electronic transference through the galvanic cell, thus avoiding a possible error of previous electrochemical investigations. The method is based on the determination of the oxygen chemical potential that, via the sodium oxide activity of an auxiliary phase, is reproducibly related to the sodium chemical potential established by the heterogeneous phase equilibrium Na2ZrO3(s)/ZrO2(s)/O-2. The standard Gibbs free energy of formation of Na2ZrO3 yields (450 degrees C to 650 degrees C): Delta(f)G(Na2ZrO3)(degrees) [kJ/mol] = - 1711.828 +/- 53.21 + (310.974 +/- 66.52) . 10(-3) . T/K. This data is by about 55 kJ/mol more negative than the findings of previous electrochemical studies, thus proving the significance of electronic transference therein. (c) 2006 Elsevier Ltd. All rights reserved.