Abstract: An elastically deformable plastic cap for containers such as bottles having necks, such containers containing materials under pressure, such as aerated liquids. The cap consists of two cylinders disposed concentrically one within the other. The outer cylinder has a sealing ring with a sealing edge at its lower end and the inner cylinder is closed at its bottom end with a diaphragm. The cylinders are connected at their upper ends by means of a ring. The bottom surface of the diaphragm and the bottom surface of the sealing ring are disposed in a first common plane; the sealing edges on the inner surface of the sealing ring and the outer surface of the inner cylinder respectively and the upper surface of the diaphragm lying in a second common plane, the second plane being parallel and above the first common plane.