Abstract: A closure cap is provided for an opening in a vessel. Inner and outer seatings extend around the opening. The closure cap has a body with attachment means to enable the cap to be releasably secured in the opening, and the body also has an inner seal for engaging the inner seating and an outer seal for engaging the outer seating. The cap is characterized by the fact that at least one of those seals comprises an outwardly directed flange of a flexible and resilient plastics material supported at its inner edge and yielding resiliently when in use. The flange is urged towards the associated seating due, at least in part, to the innate resilience of the material from which it is made.