Abstract: In a process for heating a substance which is substantially non-conducting electrically, at least one layer of the substance is disposed within a cavity between two plates of a metal which is a good conductor of electricity. Each plate lies in intimate contact with a respective one of the two larger surfaces of the substance. The resulting assembly of the substance and the plates is disposed at a predetermined minimum distance from the walls of the cavity. The substance is then exposed to electromagnetic radiation in the decimeter wavelength band at a predetermined power level and for a predetermined time.