Abstract: A sealable pressure vessel 5 has a closure member 8 held on by means of a bayonet fixing member which is movable between two positions while retaining the mechanical linkage between the vessel 5 and the closure member 8 forming a container 1. The bayonet fixing comprises a vessel lug 19 which co-operates with a stepped slot 18 in a locking ring 9 secured to the closure member. In one position of the lug 19 in the slot 18 the closure member is sealed on the vessel and the container is closed to atmosphere while in another position the seal is broken and the container is open to atmosphere. The closure member 8 incorporates a sliding valve 29 which has a spring urged piston 32 carrying a locking pin 33 to lock the lug 19 within the slot 18 when the container is closed to atmosphere.