The iodine-sulfur thermochemical water-splitting process (I-S process) is regarded as one of the most promising high efficient, CO(2) free, and large scale hydrogen production methods. The HI concentration subsection of the HI decomposition section (HI(x) section) is the most energy consuming portion of the whole process. To optimize the I-S process and improve hydrogen production efficiency, it is significant to extensively investigate and utilize the thermodynamic properties of the components in HI(x) section. This paper summarizes and reviews the thermodynamic properties of the components in HI(x) section of the I-S process. The phase equilibrium property and the values of thermodynamic functions of the binary mixtures, i.e., I(2)-H(2)O, HI-H(2)O, HI-I(2) and the ternary system HI-I(2)-H(2)O are summarized and compared. In addition, the tri-iodide complexation equilibrium is demonstrated and several equilibrium constant expressions provided by different literature are compared. Finally, the thermodynamic models for the ternary system are summarized. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.