The HI, ternary system (H2O-HI-I-2) is the latent source of hydrogen for the Sulfur-Iodine thermo-chemical cycle. After analysis of the literature data and models, a homogeneous approach with the Peng-Robinson equation of state used for both the vapor and liquid phase fugacity calculations is proposed for the first time to describe the phase equilibrium of this system. The MHV2 mixing rule is used, with UNIQUAC activity coefficient model combined with of hydrogen iodide solvation by water. This approach is theoretically consistent for HIx separation processes operating above HI critical temperature. Model estimation is done on selected literature vapor-liquid, liquid-liquid, vapor-liquid-liquid and solid-liquid equilibrium data for the ternary system and the three binaries subsystems. Validation is done on the remaining literature data. Results agree well with the published data, but more experimental effort is needed to improve modeling of the HI, system. (C) 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.