The thermochemical water splitting cycle is an environmentally attractive way to produce hydrogen without using fossil fuels. Among hundreds possible cycles, the sulfur – iodine (IS) cycle is a promising one, expected to become a major source of hydrogen production from nuclear or solar energy. The actual challenge with this cycle remains in describing the thermodynamics of the HIx system (H2O – HI – I2) which is a strong electrolyte system, extremely complex because of various phase behavior that occurs (vapor – liquid – liquid – solid) over the large range of pressure and temperature spanned in the process. A review of existing models shows that two approaches can be adopted to deal with phases equilibrium ; (1) homogeneous one in which the same equation of state is applied to different phases in equilibrium and (2) combined approach where the vapor phase is described by an equation of state and the liquid one by a coefficient activity model. The homogeneous approach would be effective for the high pressure HIx system but cannot capture alone the strong non ideality of the liquid phase. That may explain why the current HIx model written by Neumann has been done based on an activity coefficient model coupled with a solvatation formalism proposed by Engels. Unfortunately, activity coefficient models fail for critical compounds in mixtures as is the case for HI under temperature conditions expected in the process. Based on this analysis, we propose a novel modeling of the HIx system that matches all the constraints enounced.