<p>Pressurised air at a temperature suitable for its separation by cryogenic distillation is passed into a double distillation column 2 comprising a higher pressure column 4 and a lower pressure column 6 and a condenser-reboiler 8. Oxygen-rich liquid is withdrawn from the column 4 through an outlet 12, is sub-cooled in heat exchanger 14 and is separated in column 6 in nitrogen vapour and liquid oxygen. Argon-enriched fluid is withdrawn from the column 6 through outlet 50 and argon is separated from it in a further distillation column 16 having a condenser 18 which is refrigerated by the oxygen-rich liquid. In a typical embodiment of the invention the oxygen-poor liquid is withdrawn though outlet 28 from the column 4 and is expanded through valve 30 into phase separator 32. The vapour phase from the separator 32 is used to provide extra refrigeration to the oxygen-rich liquid by counter current heat exchange in the heat exchanger 14 and is then either taken as product or expanded in a turbine to create refrigeration. Part of the liquid phase from the separator 32 is used as reflux in the column 6 while the remainder is used to provide extra cooling for the condenser 18.</p>
