Thermodynamic data for the solubility and formation of copper(I)and copper(II) species are required to model innovative designs of the electrolysis cells used in the thermochemical copper-chloride hydrogen production process. Cumulative formation constants of Cu2+(aq) complexes with Cl-(aq) were determined by Principal Component Analysis of UV-spectra, obtained over a very wide range of solution compositions and temperature, coupled with an appropriate model for activity coefficients of the solution species. Shortcomings in the existing database and future work to extend these studies to higher temperatures and much higher copper concentrations are discussed. Crown Copyright (C) 2009 Published by Elsevier Ltd on behalf of Professor T. Nejat Veziroglu. All rights reserved.