A humidification chamber (10) for use in a breathing circuit to humidify gases before inhalation, the humidification chamber being adapted to contain a volume of liquid (20), and comprising a gas inlet port (16), a gas outlet port (16), a fluid inlet (30) including a primary valve seat. The humidification chamber also has a valve having an actuating member (42) that is movable in response to a change in the level of liquid within the humidification chamber between an open configuration in which the actuating member is disengaged from the primary valve seat such that liquid is able to flow through the fluid inlet into the humidification chamber when the liquid within the humidification chamber is below a predetermined acceptable level, and a closed configuration in which the actuating member is engaged with the primary valve seat such that liquid is prevented from flowing through the fluid inlet into the humidification chamber when the liquid within the humidification chamber is at or above the predetermined acceptable level. The fluid inlet (30) includes a secondary valve seat, and the actuating member (42) is deformable in response to an increase in the level of liquid within the humidification chamber above the predetermined acceptable level to a deformed configuration in which the actuating member engages the secondary valve seat such that liquid is prevented from flowing through the fluid inlet into the humidification chamber.