This paper examines the transient chemical decomposition of solid particles in a multiphase flow consisting of molten and gaseous phases at high temperature and low Reynolds number. The mass, energy, momentum and chemical reaction equations are solved for a particle immersed in a viscous flow of uniform properties. Numerical solutions are obtained and the results are validated with experimental data of samples involving the chemical decomposition of the mixture of CuO and CuCl2 particles in a molten salt. The analysis provides new insight into the implications of the rate of reaction vs. heat and mass transfer in a complex gas-solid-molten salt multiphase reacting system, as well as the impact on hydrogen yield in the thermochemical copper-chlorine (Cu-Cl) cycle of hydrogen generation. (C) 2012 Elsevier Ltd. All rights reserved.