Cyclic stability is a key factor in the design of thermochemical heat storage systems. Because carbon dioxide (CO2) may react with heat storage materials and lead to a decrease in energy storage efficiency, the effect of CO2 on Ca(OH)(2)/CaO and Mg(OH)(2)/MgO systems is investigated in this study. The experimental results show that CO2 reacts with CaO in the water vapor that appears during the heat release process. Therefore, in the design of Ca(OH)(2)/CaO systems, CO2 should be cleared from the system. The results from Mg(OH)(2)/MgO systems show that CO2 only slightly reacts with MgO and Mg(OH)(2) during heat storage and release processes. This study indicates that carbonic acid (H2CO3) could easily react with CaO/Ca(OH)(2) to form CaCO3 during heat release processes. Generally, the remaining CO2 reduces the reversibility of Ca(OH)(2)/CaO systems but has only a slight influence on Mg(OH)(2)/MgO systems. In addition, the experimental results show that carbonate shell does not exist in rehydration for both of the CaO/MgO samples, but the influence of CO2 on the entire process increases after each cycle. (C) 2017 Elsevier Ltd. All rights reserved.