A method of, and apparatus for, providing improved separation of gases in a gas mixture using particularly, but not exclusively, permeable membranes (28). In passing over a membrane (28), a portion of a gas mixture in intimate contact with the membrane wall experiences a change in density as components of the gas selectively diffuse through the membrane (28). In particular, the present invention uses gravity or other force to act on the density difference created by diffusion to assist in a) "peeling" (away from the wall) the gas which has already undergone substantial diffusion: b) keeping this gas away from the diffusion surface, once it has left; c) removing the gas from the membrane (28) more quickly; and d) slowing gas which has had less exposure to the diffusion process and moving it towards the membrane wall. Embodiments of the invention are described.