The dehydration and hydration behaviors of Mg-Co mixed hydroxides and the effect of LiCl addition were studied to develop a new material for chemical heat storage. The formation of the mixed hydroxides and LiCl addition were expected to reduce the dehydration temperature of authentic magnesium hydroxide; this temperature corresponds to that of heat-storage operation. That the dehydration temperature of Mg-Co mixed hydroxides was found to be approximately 280 degrees C, which was lower than that of authentic magnesium hydroxide. The mixed hydroxides showed higher hydration reactivities than authentic magnesium oxide at 110 degrees C with 57.8 kPa of water vapor after dehydration at 300 degrees C. The dehydration temperatures of Mg-Co mixed hydroxides were lowered by LiCl addition. LiCl-added Mg-Co mixed hydroxides showed higher hydration reactivities than the usual Mg-Co mixed oxides after dehydration at 250 degrees C. The heat-output performance of LiCl/Mg0.95Co0.(05)(OH)(2) was estimated as 715 kJ kg(-1) after dehydration at 250 degrees C. These results indicated that Mg-Co mixed hydroxides can be used to utilize industrial waste heat between 250 degrees C and 300 degrees C.