The sorption behavior of dehydrated magnesium sulfate in porous glasses with pore diameters from 1.7 µm to 7 nm was investigated by using isothermal calorimetry. The kinetic hindrance of the formation of the thermodynamically stable product MgSO 4 ·7H 2 O was not overcome at 85 % RH and 30 °C in the material with the largest pores. In contrast, in pores below 173 nm the high water uptake clearly indicates the formation of a solution. The released heat (Q r) can be calculated as the sum of the heat of hydration (Q hyd), the heat of condensation (Q c) and the heat of solution (Q sol). The water uptake and the overall released heat both increase with decreasing pore size. Although high energy densities up to 0.95 GJ/m³ were found for the material composed of Vycor glass (d m = 7 nm) and MgSO 4, this composite does not seem to be suitable as heat storage material due to the formation of a solution and the high humidity of 85 % RH required for the release of heat. Porous carbon was used as an alternative host material for MgSO 4. Isotherms of the water vapor uptake of a porous carbon composite with three different loads of MgSO 4 ·H 2 O were determined and are discussed in this paper. An energy density of 0.74 GJ/m 3 can be achieved without formation of a solution at 72 % RH.