Query electrodes are provided for monitoring the electrical impedance of tissue as it is treated with electrosurgical energy. In one embodiment, based on a predicted model of tissue impedance and a number of initial impedance readings, the impedance at which tissue treatment is completed is predicted. More particularly, a minimum impedance level is measured and a function of the minimum impedance is used to determine impedance at which coagulation is completed. A control device is provided for bringing the output of the generator within an optimum range based on a system load curve. In one embodiment the impedance monitoring device is used in conjunction with a bipolar electrosurgicalinstrument. Preferably, the instrument comprises electrically opposite therapeutic electrodes, each located on one or more tissue engaging surfaces for engaging tissue to be treated, and electrically opposite query electrodes, located on one or more tissue engaging surfaces. The therapeutic electrodes provide therapeutic energy to the tissue while the query electrodes provide a lower voltage electrical sensing energy for the purpose of measuring tissue impedance at various stages, e.g. before, during, or after tissue treatment.