The design and optimization of the separation units of the sulfur-iodine (SI) cycle process for hydrogen production requires a comprehensive analysis of the phase behavior of complex electrolyte solutions. Here, we apply the SAFT-VRE equation of state for electrolyte solutions to represent the phase equilibria of the HIx mixture (HI + H(2)O + I(2)) encountered in the reactive distillation column of the SI process. The phase equilibria of the binary mixtures (I(2) + H(2)O, HI + I(2) and H(2)O + HI) are accurately represented with the SAFT-VRE model. One key advantage of the SAFT-VRE model is the possibility to predict phase equilibria at high pressures and the densities of the coexistent phases, as well as the mean activity and osmotic coefficients of HI. (C) 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.