A bipolar surgical instrument (1) comprises a body (2), first and second opposed jaws (18, 20) located at the distal end of a shaft (10), the first jaw (18) being movable with respect to the second jaw (20) between an open position in which the first and second jaws (18, 20) are spaced apart from one another, and a closed position in which the first and second jaws (18, 20) are adjacent one another. The first and second elongate jaw members (18, 20) have respective first and second electrodes. A controller is operable to determine a mode of operation depending upon a measurement of impedance between the first and second jaws.
