A new effective processing for polychlorinated biphenyls (PCBs) has been recently developed. PCBs become harmless by dechlorination using 2.45 GHz-microwave (MW). To investigate such chemical reaction by MW irradiation, the dielectric loss of pure PCBs solutions and an isomer mixture in MW band is measured. Some PCBs and isopropyl alchol, added as a source of hydrogen supply have high dielectric loss in MW band. This suggests that MW heats PCBs and isopropyl alchol, and then the temperature of the solvent consisting mainly of insulating oil rises by the thermal conduction. Copyright © 2009 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.