CaO hydration and Ca(OH)(2) decomposition cycles can be used to supply or transport energy (heat). The reaction rates and properties of these materials are important parameters for the design of a reactor. In this work, the rates of CaO hydration and Ca(OH)(2) decomposition were measured continuously over 20 hydration/decomposition cycles by means of a high-pressure thermogravimetric apparatus. CaO was completely converted to Ca(OH)(2) in all cycles, but the CaO hydration rate decreased with increasing number of cycles. Ca(OH)(2) was decomposed to CaO completely in all cycles. The extent and rate of conversion for Ca(OH)(2) decomposition were not influenced by increasing cycle number. Eutectic melting of CaO and Ca(OH)(2) was not observed on the particles after CaO hydration or after Ca(OH)(2) decomposition. The compressive strength of CaO particles decreased with increasing cycle number, and the average compressive strength of CaO particles after 20 cycles was about 16.5 kg/cm(2).