Mg-based metal hydride systems are potential high temperature heat storage media. In this paper the features and possibilities of the systems Mg/MgH2, Mg-Ni/Mg2NiH4, Mg-Fe/Mg2FeH6 and Mg-Co-H are discussed. All the materials show cyclic stability in certain temperature ranges. The thermal energy which is released by these systems covers a temperature range from 250 degrees C to 550 degrees C in which high thermal energy densities of up to 2257 kJ/kg are reached. (C) 2000 International Association for Hydrogen Energy.