A closure device with a fluid chamber containing an additive. The additive can be introduced and mixed into a liquid on the container by the closure device. The closure device includes a cap member having a fluid chamber and a housing having a plug member. The cap member is provided with a primary engagement member which engages with a corresponding primary engagement member provided on the housing to allow the cap member to be lifted relative to the housing from a closed position in which the plug member closes an aperture in the fluid chamber to an open position in which the plug member is at least partially withdrawn from the aperture, thereby allowing the additive to pass from the fluid chamber to the liquid in the bottle. The closure device enables mixing of the additive and liquid in the bottle without opening the closure.