Abstract: In an expansion tank with a bladder-type diaphragm inside the tank, the diaphragm is provided on the outside with an annular open-holding element fixed to the diaphragm. The annular open-holding element may be positioned between two inwardly extending ribs disposed on the inside wall of the expansion tank. The ribs lie at distance from each other and extend preferably essentially perpendicular to the center line of the tank. The bladder-type diaphragm is preferably made up of two parts being attached to each other with their edges by means of an open-holding ring with a U-shaped cross-section. The edges of the diaphragm parts are accommodated in the U-shape and clamped together by the legs thereof. In the collapsed state the shape of the diaphragm can be controlled so that an optimal shape as regards the strain in the material of the diaphragm is obtained.