Abstract: A double seat valve includes a valve housing and two valve closure members movable in the housing co-axially and relative to each other between an open and closed position. A leakage space defined between the closure members by a drain outlet. One of the closure members is a slide piston having an end portion sealingly received in a recess provided in the other closure member. The wall of the recess has an annular groove which permits a seal cleaning flow of liquid between the surfaces of the seal provided on one of the closure members and the annular groove into the leakage space.