A defrosting system includes an RF signal source, one or more electrodes proximate to a cavity within which a load to be defrosted is positioned, a transmission path between the RF signal source and the electrode(s), and an impedance matching network electrically coupled along the transmission path between the output of the RF signal source and the electrode(s). The system also includes measurement circuitry coupled to the transmission path and configured to measure one or more parameters that include voltage, current, forward signal power, reflected signal power, and S11 along the transmission path. A system controller is configured to monitor the measurements, and to modify operation of the system when a rate of change of any of the monitored parameter(s) exceeds a predetermined threshold. The impedance matching network may be a single-ended network or a double-ended network.