An electrosurgical instrument is provided for the treatment of tissue, the instrument comprising an instrument shaft (10) having a longitudinal axis, and an electrode assembly at one end of the shaft. The electrode assembly comprises a first tissue treatment electrode (11), a second tissue treatment electrode (14), and a third electrode (25) which is electrically insulated from the first and second tissue treatment electrodes by means of an insulation member (12). The first and second tissue treatment electrodes (11, 14) each have an exposed surface for treating tissue, the exposed surface of the first tissue treatment electrode (11) being such as to treat tissue disposed on the longitudinal axis, and the exposed surface of the second tissue treatment electrode (14) being such as to treat tissue disposed laterally of the longitudinal axis. The instrument has a first set of connections (62A, 62B) by which the first tissue treatment electrode (11) can be placed in circuit with the second electrode (14) such that, in use, a current path is established therebetween. The instrument has a second set of connections (62B, 62C) by which the second tissue treatment electrode (14) can be placed in circuit with the third electrode (25) such that, in use, a current path is established therebetween.長手方向軸を有する器具シャフト（１０）と、該シャフトの一端にある電極アセンブリとを備える組織処置用電気外科用器具を提供する。電極アセンブリは、第１の組織処置電極（１１）と第２の組織処置電極（１４）と第１及び第２の組織処置電極から絶縁部材（１２）によって電気的に絶縁された第３の電極（２５）とを備える。第１及び第２の組織処置電極（１１、１４）は各々組織を処置するための露出した表面を有し、第１の組織処置電極（１１）の露出した表面は長手方向軸上に位置する組織を処置し、第２の組織処置電極（１４）の露出した表面は長手方向軸の側方に位置する組織を処置する。器具は、使用に際して第１の組織処置電極（１１）と第２の電極（１４）との間に電流経路が確立されるように第２の電極を有する回路内に第１の組織処置電極を配置させる第１の組の接続部（６２Ａ、６２Ｂ）を有する。器具は、使用に際して第２の組織処置電極（１４）と第３の電極（２５）との間に電流経路が確立されるように第３の電極を有する回路内に第２の組織処置電極を配置させる第２の組の接続部（６２Ｂ、６２Ｃ）を有する。
