Abstract: Air is separated in a single fractionation column (116) to produce a top nitrogen gaseous fraction (142) and a bottom liquid fraction (120) containing less than (80) mole percent of oxygen. A liquid nitrogen product (136) is also produced. The necessary refrigeration is created by expansion with the performance of external work of firstly a stream of compressed air in an expansion turbine (114) and of secondly a stream of vaporised bottom fraction in an expansion turbine (128). At least part of the feed to the column (116) comes from the turbine (114). The outlet pressure of the turbine (114) is essentially the gaseous nitrogen product pressure. A double fractionation column may be used instead of the single fractionation column (116).