A method of separating air in which a compressed air stream is divided into first and second subsidiary streams. The first subsidiary air stream is cooled by heat exchange to a temperature suitable for its separation by rectification and introduced into the higher pressure stage of a double rectification column. The second subsidiary air stream is further compressed and then at least part of it is cooled by heat exchange to a first intermediate temperature below ambient temperature but above those temperatures at which the double rectification column operates. The thus cooled second subsidiary air stream is expanded in a first expansion turbine and is withdrawn therefrom at a second intermediate temperature below the first intermediate temperature but above those temperatures at which the double rectification column operates. After withdrawal, the second subsidiary air stream is introduced into a second expansion turbine where it is further expanded. After the further expansion, the second subsidiary air stream is introduced into the lower pressure rectification stage of the double rectification column. Air is separated in the double rectification column into oxygen and nitrogen. Oxygen and nitrogen streams are withdrawn from the said lower pressure stage and a part of at least one of the oxygen and nitrogen is produced as a liquid product.