Abstract: Disclosed is a closure means for a pressure vessel, said pressure vessel comprising a body portion and a lid portion, the closure means comprising: securing means attached to the lid portion for releasably securing the body and lid portions together to form a subtantially gastight enclosure, the securing means permitting a limited degree of movement between the body and lid portions from a sealing position to a venting position when the pressure within the vessel exceeds the external pressure by a predetermined amount thereby permitting venting of excess pressure, the closure means comprising biasing means acting to bias movement of the body and lid portions from the venting position to the sealing position after venting of excess pressure, and pressure vessels comprising said closure means.