A closure device (2) for a fluid vessel (1) comprises a bistable valve arrangement which includes first and second valve rim portions (4, 5) and a valve member (3) engageable with either of the rim portions (4, 5) respectively to define an open position in which fluid (9) can exit the vessel (1) and a closed position in which fluid (9) is substantially prevented from exiting the vessel (1). The valve is arranged to close automatically when an impulse is exerted against the valve member (3). This can occur for example when the vessel (1) is knocked over and the fluid (9) therein exerts pressure against the valve member (3).