The calcium oxide hydration/dehydration reaction is proposed as a suitable reaction couple for thermochemical energy storage systems. However, limited work has been reported on the reaction kinetics of CaO/Ca(OH)(2) under appropriate operation conditions for storage applications involving fluidized beds. This study focuses on the effect of temperature, partial steam pressure, and particle size upon the intrinsic hydration and dehydration reaction kinetics when natural materials are used. The experimental data have been fitted satisfactorily to a shrinking core model for both hydration and dehydration reactions, at reaction temperatures between 400 and 560 degrees C and partial steam pressures between 0 and 100 kPa. The reaction rates measured are higher than those previously reported in the literature. In the case of large particle sizes of natural material, particle attrition has been detected indicating the need to develop more suitable materials for thermochemical energy storage applications.