Apparatus for changing the concentration of a selected gas in a liquid, the apparatus comprising a flow chamber through which the liquid is passed and which comprises a wall comprising a planar separation membrane, the separation membrane being substantially impermeable to the liquid and permeable to the selected gas, characterized in that the separation member extends beyond the flow chamber and provides a seal between components of the apparatus. The apparatus is particularly useful for degassing liquids, for example HPLC eluents and analysis samples.