An electrosurgical instrument for the treatment of tissue in the presence of an electrically-conductive fluid medium comprises an instrument shaft (10), and an electrode assembly (12) at one end of the shaft. The electrode assembly (12) comprises a tissue treatment electrode (14) and a return electrode (18) which is electrically insulated from the tissue treatment electrode by means of an insulation member (16). The tissue treatment electrode (14) has an exposed surface for treating tissue, and the return electrode (18) has a fluid contact surface so as to define, in use, a conductive fluid path that completes an electrical circuit between the tissue treatment electrode and the return electrode. The tissue treatment electrode (14) is provided with at least one aperture (14a) through which vapor bubbles and/or particulate material can be aspirated from the region surrounding the tissue treatment electrode, and the tissue treatment electrode and the insulation member are disposed such that there is a gap therebetween forming an additional suction channel (25).
