Abstract: In order to obtain a more homogeneous energy absorption and preset maximum temperatures in an apparatus for radiating microwave energy into water-containing material or material mixed with water, which is introduced into a cavity resonator subjected to microwaves by one or more microwave generators, a pressure and/or temperature sensing device being included, the proposal is put forward that a sealable container be provided for receiving the material to be treated, which should be transparent to microwaves and resistant to gases and pressures, and whose interior should be enclosed by the cavity at a given distance, and that the pressure and/or temperature sensing device be connected with the interior of this container, and further that a device be provided for control of the energy output of the microwave generator, which device should control and maintain constant a preset interior pressure deviating from atmospheric pressure, and which should be connected to the pressure and/or temperature sensing device.