Abstract: A two-phase thermosyphon having an evaporator where heat can be absorbed ompanied by the vapourisation of liquid contained therein, a condenser where heat can be given up by condensation of vapour and a condenser reservoir associated with the condenser and which can retain condensate from the condenser, is provided with means, such as a vapour lift pump having a controllable flow connecting tube with a lower end in the condenser reservoir, for controlling the level of liquid in the condenser reservoir which determines the rate of heat transfer through the thermosyphon.