A magnesium sulfate/polymer composite has been synthesized as active material for seasonal, thermochemical heat storage. It was found that the salt can store heat in the process of hydration/dehydration and the porous polymer matrix contributes due to adsorption and desorption of water. The composite shows high water uptake and a high gravimetric as well as volumetric heat storage capacity. The presented concept can be adapted to different salts and particle sizes of the respective salt within the matrix to investigate possible effects of particle size on heat of hydration and kinetic effects.