Attapulgite granulate impregnated with mixtures of MgSO4 and MgCl2 hydrates was investigated for suitability as a composite thermal energy storage material. These mixtures were chosen because of the very different deliquescence relative humidities of the salts. The thermochemical heat storage of these materials was characterized by measurements of isothermal heat of sorption and thermogravimetry (TG). The salt mixtures showed a different sorption behavior compared to pure MgSO4. The reduction of the deliquescence relative humidity of the mixture by the partial substitution of MgSO4 by MgCl2 increases the capacity of condensation and therefore the released heat. The energy density of the composite heat storage material containing a salt mixture of 20 wt% MgSO4 and 80 wt% MgCl2 was 1590 kJ/kg measured by calorimetry (at 30 degrees C/85% RH) with a desorption temperature of 130 degrees C. (C) 2010 Elsevier B.V. All rights reserved.