The reversible hydration of CaO to Ca(OH)(2), was investigated in open and closed systems. The experimental results were compared with that of the Semi-closed system reported in the previous paper. The pressure for reaction in the open system was higher for hydration and lower for dehydration than in the closed system at the same temperature. The final hydration conversion in the closed system at given reaction temperature was lower than in the open and semi-closed systems. The specific surface area of CaO obtained by the dehydration in the semi-closed system was about 1.2 times greater than that in the closed system. A model previously proposed for semi-closed system was found to be applicable also to the closed and open systems.