A model for the phase equilibria and energetics of the HI-I-2-H2O-H-2 system has been developed for use in process simulations of reactive distillation to form H-2 from HI in the Sulfur-Iodine cycle for thermochemical decomposition of water. Comparisons with data and other models demonstrate that this approach is uniquely successful in describing the phase equilibria and properties of this complex system. In particular, it is able to follow the pressure minimum of the HI-H2O binary and HI-I-2-H2O ternary when x(HI) approximate to 0.15; the dramatic increase of pressure and HI vapor composition, as well as formation of two liquid phases with greater amounts of HI; and the emergence of a single liquid phase in the ternary, even though binaries of HI and of I-2 with H2O are only partially miscible. The model incorporates two novel speciation reactions and the Electrolyte-NRTL excess Gibbs energy model with parameters that cover wide ranges of temperature, pressure and composition. The model is formulated so that process simulation, design and optimization may be done reliably to explore feasibility and innovations of the S-I process. (C) 2009 Elsevier B.V. All rights reserved.