Lithium chloride (LiCl)-modified magnesium hydroxide (Mg(OH)(2)) is a material used for chemical heat pumps. LiCl-modified Mg(OH)(2) is able to store thermal energy at around 250 degrees C. LiCl-modified Mg(OH)(2) was examined using 1, 7, and 105 runs of a cyclic dehydration and hydration operation. The sample was dehydrated at 250 degrees C and hydrated at 110 degrees C. It was found that the hydration conversion, which corresponds to the heat output performance, decreased from 81.1 to 65.7%. There were no LiCl needle-like crystals on the sample before the cyclic operation. On the other hand, after 105 runs of the cyclic operation, the sample showed LiCl needle-like crystals. The reason for the decrease in the reactivity of the LiCl-modified Mg(OH)(2) could be a change in the surface state.