A new candidate for a chemical heat storage material used in a magnesium oxide-water chemical heat pump at medium- temperature (200-300 degrees C) was developed. The new composite, named EML, was fabricated by mixing pure magnesium hydroxide (Mg(OH)(2)) with lithium bromide (LiBr) and expanded graphite, which are employed as reactivity and heat transfer enhancers, respectively. The effects of mixing mole ratios of LiBr-to-Mg(OH)(2), alpha, on both dehydration and hydration were kinetically investigated by a thermogravimetric method. It was experimentally demonstrated that the reacted mole fractions of dehydration and hydration increased with increasing alpha value. Further, the activation energy, E-a, of the dehydration process was significantly reduced for the EML with high alpha. Therefore, it was concluded that the E-a values determined here are dependent on alpha. Further, the EML composite showed cyclic ability with repeat reactions over five cycles. Thus, this composite can be used as a chemical heat storage material because of its ability to store heat at medium-temperatures ranging from 220 to 300 degrees C.