Abstract: A heat exchanger comprises an evacuated reaction chamber (2, 2?, 23, 33) containing at least one reaction medium and a storage chamber (25, 35) containing an activation medium which reacts with the reaction medium in such a way as to modify the temperature. The reaction chamber and the storage chamber can be brought into a communicating relationship, whereby the reaction medium and the activation medium are contacted with each other and the temperature-modifying reaction is triggered. The reaction chamber (2, 2?, 23, 33) is defined by walls made of a vacuum-tight material (3), which walls are kept at a distance from each other, at least in sections, by supporting bodies (4, 7, 10), with the supporting bodies (4, 7, 9, 10) keeping free transport paths (5, 8) for the reaction medium (11) inside the reaction chamber. The vacuum-tight material (3) forming the walls of the reaction chamber is designed as a flexible film, preferably a composite film.