The invention relates to a heat transfer medium comprising a mixture containing elemental sulphur and at least one additive and a use of the heat transfer medium. A heat transfer medium comprising a mixture containing elemental sulphur and at least one additive is indicated. The heat transfer medium is characterized in that the additive comprises at least one halogenated hydrocarbon. The halogenated hydrocarbon is, in particular, a chlorinated and/or brominated paraffin. The heat transfer medium is used for reversible energy storage. The heat transfer medium is preferably used for operating a solar thermal power station for converting solar energy into electric energy. Sunlight is converted into heat energy of the heat transfer medium.