A bipolar surgical instrument comprises a body, first and second opposed jaws located at the distal end of a shaft, the first jaw being movable with respect to the second jaw between an open position in which the first and second jaws are spaced apart from one another, and a closed position in which the first and second jaws are adjacent one another. The first and second elongate jaw members have respective first and second electrodes. A controller is operable to determine a boiling point for tissue between the jaws using a measure of impedance therebetween.