Electro-electrodialysis (EED) has been applied to thermochemical water-splitting hydrogen production iodine-sulfur process in order to enhance HI in a HI-I-2-H2O mixture (HIx solution). In this paper, in order to understand the behavior of the components (I-2, H+, I- and H2O) of the HIx solution in the membranes on absorption equilibrium, each component absorbed in a styrene-grafted poly-(ethylene-co-tetratluoroethylene) (ETFE-St) membrane and Nation 212 were evaluated. The behavior of each component in the membrane was as follows. The I-2 content in the ETFE-St membrane increased with an increase in the I-2 concentration of the immersed HIx solution, whereas I-2 absorption by Nation 212 was minimal. The strong affinity of the ETFE-St membrane for I-2 was attributed to the formation of a charge-transfer complex with electron-donating aromatic groups. The ETFE-St membrane absorbed I- (also H+ under the neutral condition) in a HI-molality-dependent manner, whereas I- (H+) absorption by Nation 212 was negligible. This difference was attributed to the I-3(-)-complex formation in the ETFE-St membrane, which was precluded in Nation 212. The absorption of H2O was greater for the ETFE-St membrane than for Nation 212. Incorporation into Nation 212 would not greatly stabilize H2O while stabilizing H+ by a strong electrical attraction. These results accordingly clarified the affinity of the membranes for the HIx solution based on the excess Gibbs free energy calculated from the measured composition data and the estimated activity coefficients. The excess Gibbs free energy for I-2 was larger in the membranes than in the immersed solution, indicating a destabilization of I-2 in the membranes. Importantly, this difference agreed with the energy change estimated under the assumption that I-2 could form I-3(-) or the charge-transfer complex. The affinity for I-2 as well as between the sulfonic acid groups and H+ or H2O is a factor determining the absorbability of the HIx solution. (C) 2014 Elsevier By. All rights reserved.