The durability of the magnesium oxide/water reaction system under repetitive reaction conditions was examined experimentally to demonstrate the operability of a magnesium oxide/water chemical heat pump. Changes in the reactivity in the hydration of magnesium oxide, which was the most sensitive process in the repetitive reaction, were measured at hydration temperatures from 90-170 degrees C and pressures from 31.2-70. 1 kPa. A lower temperature and a higher pressure for the hydration reaction were effective in maintaining reactivity. The effect of different reaction conditions experienced during repetitive cycles to the standard repetitive cycle was discussed. From microscopic observations, nucleation in primary particles of the material induced by the structure change during repetitive reaction was an important factor in the enhancement of durability.