A cap is described for closing a liquid beverage container for allowing the beverage to be sucked out of the container through a spout, while preventing spillage when no suction takes place. A demand valve is incorporated into the cap, which has an inlet port communicating with the interior of the container, a discharge port communicating with the spout and a control port communicating with the ambient atmosphere through a hole in the cap. The valve has a valve seat and a closure element controlling the flow from the inlet port to the discharge port urged to move into an opening position in dependence on the excess of the control pressures over that in the discharge ports, this movement being in the direction opposed to the flow and urged by the pressure in the container in a direction to seal against the valve seat. In the invention, the demand valve is formed by two members that are mounted on the inner surface of the cap. The first member is rigid and defines the valve inlet port and the valve seat. The second member incorporates the valve closure element and a resilient membrane which includes the valve closure element and serves as a pressure sensitive diaphragm. The second member also seals against the first member, around the hole in the cap and around the spout.