A magnesium oxide/water chemical heat pump using a 10-W class packed bed reactor was examined experimentally to evaluate the thermal performance of the heat pump. A developed reactant having high durability for repetitive operation was packed in the cylindrical reactor. The magnesium oxide hydration was studied under reaction pressures up to 405 kPa, which were higher than one of previous work. Heat generation at around 100–240°C was measured by the hydration under the heat amplification mode operation. The magnesium hydroxide dehydration was capable to store the surplus exhaust heat from a cogeneration system and surplus night-time electricity. The heat storage density of the heat pump was sufficiently competitive with that of a conventional sensible heat water storage system. The heat pump was expected to be applicable for load leveling in a cogeneration system by chemically storing surplus heat during low heat demand and supplying heat during peak demand.