Apparatus for desorbing gas from a liquid comprises a deflector for imparting a helical movement to the liquid, a nozzle forming the outlet of the deflector, a stepped desorption tube extending upwardly from the nozzle and a receptacle enclosing the outlet end of the desorption tube. A vacuum pump is connected to the receptacle to reduce pressure within the receptacle and feed and extraction pumps are respectively connected to the deflector and the receptacle to pass the liquid through the desorption tube and to remove de-gassed liquid from the receptacle. The desorption tube has an inlet end with a larger cross-sectional area than the outlet of the nozzle and a cross-section which increases stepwise along the length of the desorption tube. Liquid issues from the nozzle in the form of a divergent jet which extends radially outwards into contact with successive sections of the desorption tube which are bounded by a stepwise change in cross-sectional area to thereby seal totally enclosed spaces between the jet and said sections of the desorption tube.