An exhalation valve (10) is disclosed for use with an exhalation port (14) of a respiratory circuit. The exhalation valve comprises a hermetically sealed gas chamber (20,120), a gas inlet (22,122) for supplying gas to the gas chamber, and a membrane (24,124) of flexible material that defines at least part of a wall of the gas chamber. The membrane is deformable by a change in the pressure differential between the gas within the gas chamber and the gas within the exhalation port between an inoperative configuration and an operative configuration in which the membrane restricts the flow of gas from the exhalation port to a greater extent than in said inoperative configuration. The membrane is inherently planar but is mounted within the exhalation valve such that deformation of the membrane to its operative configuration occurs substantially without increase of the surface area of the membrane.