The equilibrium potential across Nafion 117 in a HIx solution (HI-I-2-H2O mixture) was measured for an electro-membrane process (electro-electrodialysis, EED) in the thermochemical water-splitting iodine-sulfur process. The experimental results were correlated with a theoretical formula derived on the basis of the electrochemical reaction, the Donnan equilibrium and the Nernst-Planck equation. The obtained empirical formula could reproduce the experimental equilibrium potential within +/-0.03 V. Also, the obtained regression parameter agreed well with that calculated from the vapor liquid equilibrium data of a HIx solution reported by Hodotsuka et al. (2008) [3]. (C) 2010 Elsevier B.V. All rights reserved.