Thermal energy storage systems improve the inefficiency of industrial processes and renewable energy systems (supply versus demand). Chemical reaction is a promising way to store thermal energy because of its high energy storage density, long-term energy storage, etc. This study investigated an MgO/H2O chemical thermal storage system that stores thermal energy by decomposing Mg(OH)(2) (endothermic reaction), and supplies thermal energy by combining water vapor with MgO (exothermic reaction). Heat supply is greatly influenced by MgO properties, particularly dehydration temperature. Therefore, the equilibrium hydration fractions of MgO prepared at various dehydration temperatures were measured. Then, the relation between dehydration temperature and the equilibrium hydration fractions of MgO was determined. The equilibrium hydration fractions of MgO at various hydration temperatures and pressures were also measured. The chemical thermal storage system was inefficient at dehydration temperatures lower than 350 degrees C or higher than 500 degrees C. The efficiency of this system can be improved by increasing the hydration temperature while keeping the relative vapor pressure unchanged. (C) 2015 Elsevier Ltd. All rights reserved.