Abstract: A closure assembly for a drinking vessel comprising a lid (2) with a spout (3). A diaphragm valve (14) is mounted on a central spigot (7) of an insert which protrudes from the bottom of the spout (3) and has an enlarged head (8) which provides a sealing surface (9) which cooperates with a bead (16) around a central hole (22) in the diaphragm. The diaphragm (14) has an annular flap (18) at its periphery which normally closes a breather hole (4) in the lid (2) but opens when a user sucks on the spout (3) to allow air to enter container (1) to equalize the internal and external pressure when liquid flows past the diaphragm (14).