A closure for a keg comprises a housing and a valve element movable within the housing between closed and open positions. A lock mechanism is capable of holding the valve element in the open position, to vent gas from the keg and to thwart unauthorised re-filling after dispensing. The lock mechanism comprises: a first part movable with the valve element that comprises a lock element engageable with the housing; and a second part that is movable with the first part when the valve element moves from the closed position to the open position during filling, and thereafter is separable from the first part as the valve element returns from the open position to the closed position after filling. Separation of those parts enables the lock formation to engage with the housing to hold the valve element when the valve element returns to the open position upon dispensing.