Gas - gas mixtures can be separated either by fractional distillation of liquified gas mixtures, e.g. liquid air, or by passing them over a series of membrane separators. These membranes are built of bundles of hollow polymer tubes. Gases can diffuse through their walls. The size of the pores in the polymer walls determines which gas particles (molecules) pass through easily and which with more difficulty.

For most membrane types the permeability to oxygen gas is greater than that to nitrogen gas, seeing the differences in molecular size. The ratio of the number of oxygen to nitrogen molecules passing through the membrane determines the selectivity of the membrane, and should be at least four.

This ratio is called the selectivity or separation factor of the membrane. In practice this means that the gases must pass through several membranes in order to obtain as complete as possible a separation. The table below gives the selectivity of various polymer types for N₂/O₂ gas mixtures. The speed of separation can be increased by a combination of increased pressure (from 0.7 MPa to 10 MPa) and increased temperature (from room temperature to 95°C). The permeability of these membranes is always higher for oxygen gas than for nitrogen gas: an oxygen-gas molecule has a smaller volume (22.467 ų ) than a nitrogen-gas molecule (23.647 ų ). On the illustration, below left, the incoming air mixture is compressed (A). The gas mixture is then let in, through a regulator (B), to the membrane separator (C). At the bottom of the separator (D) oxygen enriched air is discharged. At the top of the separator almost pure nitrogen is discharged via a regulator (E).