The copper-chlorine (Cu-Cl) cycle has significantly lower temperature requirements than most other cycles of thermochemical hydrogen production. The cycle involves multiple steps which include multiphase transport processes. Thermodynamic equilibrium of ternary mixtures in the Cu-Cl cycle have important effects on the transport processes. A ternary system consisting of cuprous chloride (CuCl), hydrochloric acid (HCl) and water exists in different variations of the Cu-Cl cycle. In this paper, new correlations are developed for the solubility of the ternary system, which are essential to providing accurate boundary conditions for the transport processes. This paper examines these multi-constituent transport processes and the corresponding constituent solubilities. A new semi-empirical formulation is presented for the solubility of the ternary system. Results are given and discussed. (C) 2018 Published by Elsevier Ltd.