The thermochemical copper-chlorine (Cu-Cl) cycle for hydrogen production includes three chemical reactions of hydrolysis, decomposition and electrolysis. The decomposition of copper oxychloride for oxygen production establishes the high-temperature limit of the cycle. At 450-530 degrees C, copper oxychloride (Cu(2)OCl(2)) decomposes to produce a molten salt of cuprous chloride (CuCl, copper I chloride) and oxygen gas. Minimization of the reverse reaction and undesirable products is critical for the proper operation of the Cu-Cl cycle. This paper examines the operating conditions that disfavor the reverse reaction of the oxygen production, and the parameters that maximize the extent of the forward reaction. Experiments were designed to disperse oxygen gas into a molten CuCl bath to study its reaction at 450-500 degrees C. The composition of the products was quantified with X-ray diffraction measurements. Experimental results indicate that a high decomposition extent of copper oxychloride is obtained at equilibrium when the temperature is higher than 500 degrees C, and the oxygen pressure is below 2 bar. The thermochemistry data of the reactants and products were also determined and reported. These thermodynamic data provide a key missing gap in the understanding of the Cu-Cl cycle of thermochemical hydrogen production. The data includes the standard formation entropy, enthalpy and Gibbs free energy at different temperatures. Also, in this paper, a thermodynamic analysis is performed to investigate the reverse reaction from the aspects of spontaneity and optimization of the operating parameters. It is found that the optimal operating parameters for minimizing the reverse reaction lie in the pressure range of 1-2 bar and a temperature range of 500-525 degrees C. (C) 2011 Elsevier B.V. All rights reserved.