An electrosurgical system includes an electrosurgical instrument with a pair of opposing first and second jaw members (2), (3). A first sealing electrode (9) is located on the first jaw member (2), while a second sealing electrode (10) is located on the second jaw member (3). One or both of the sealing electrodes are provided with electrically conductive stop members (12), the one or more stop members maintaining a predetermined spacing between the first and second sealing electrodes when the jaw members are in their closed position. One or more insulating members (14), (17), (19) are present to prevent the conductive stop members from causing an electrical short between the first and second sealing electrodes when the jaw members are in their closed position. An electrosurgical generator (20) includes a controller (22) capable of supplying a sensing voltage between one or more of the conductive stop members (12) and a return electrode (9), (10), (18) present on one or other of the jaw members, the controller measuring the impedance between the one or more stop members and the return electrode in order to measure a parameter of the tissue grasped between the jaws of the instrument.
