An electrosurgical instrument has coaxial inner and outer tubular members (1,2) made of a conductive material to form a coaxial transmission line. The inner member (1) is preferably dimensioned to receive a laparoscopic tool with the working elements of the tool projecting beyond a distal end (1D) of the inner member (1). Alternatively, the inner member (1) may, itself, incorporate a working element at its distal end. The outer member (2) is connected to the inner member (1) at a distance from the distal end (1D) of the latter, and a radio frequency (r.f.) feed (4) is connected to the inner member (1) distally of the connection (7) between the inner and outer members.; Between the connections there is an r.f. isolating structure formed by or associated with the inner member (1) to present a series impedance between the connections (4,7) at an operating frequency of the instrument.