Abstract The reaction performance enhancement of a chemical heat-storage material for a magnesium oxide/water chemical heat pump was discussed. A new composite, denoted as EML, was developed by mixing pure magnesium hydroxide with lithium bromide and expanded graphite, which were employed as reactivity and heat transfer enhancers, respectively. With respect to pure magnesium hydroxide powder, the EML composite showed higher reaction rates for dehydration and hydration transformations; further, the EML could dehydrate and hydrate at the same reaction temperature (200 °C). Addition of LiBr was found to decrease the estimated activation energy in the dehydration. The heat output capacity of the EML composite calculated at a hydration temperature of 110 °C was 1405.3 kJ kg−1, which was higher than that of pure MgO. It was established that the newly developed EML composite is a promising candidate for novel chemical heat-storage materials for chemical heat pumps at working temperatures of 200–300 °C.