It is widely agreed that the most energy consuming part of the Sulphur-Iodine (S-I) thermochemical cycle for hydrogen production is represented by separation processes, especially for the HI decomposition section (HIx section). Therefore, the assessment of the real potential of the S-I cycle requires an optimization of the separation sections and, hence, a deep knowledge of the thermodynamic behaviour of the systems to be separated. In this paper, a new thermodynamic model for the electrolyte system HI-H2O-I-2 is proposed and validated on vapour-liquid (V-L) equilibrium data at atmospheric pressure. The model provides a reliable description of the phase equilibria of the ternary system and is applied for the flow-sheeting of the HIx section at atmospheric pressure along with the energy assessment of the proposed scheme. (C) 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.