The impact of exit streams containing byproducts of incomplete reactions in an integrated thermochemical copper-chlorine (Cu-Cl) cycle of hydrogen production is studied in this paper. In the hydrolysis reaction, CuCl(2)reacts with steam to produce solid copper oxychloride. If the hydrolysis reaction does not proceed to completion, particles of un-reacted CuCl(2) will be transferred to a downstream molten salt reactor where oxygen is released from copper oxychloride decomposition. Undesirable chlorine may also be released as a result of CuCl(2) decomposition together with oxygen, resulting in a mass imbalance of the overall cycle. This paper also examines the implications of incomplete hydrolysis reactions on the kinetics and thermodynamics of the oxygen reactor in the Cu-Cl cycle, specifically the spontaneity of CuCl(2) decomposition and parameters that minimize the release of chlorine. Theoretical analysis of the decomposition of a mixture of CuO and CuCl(2) is also performed in this paper. It is found that usage of copper oxychloride is preferable over a mixture of CuCl(2) and CuO in the oxygen production reaction of the Cu-Cl cycle. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.