Air is separated in a double rectification column comprising a higher pressure column and a lower pressure column, the latter operating at pressures of less than 2 bar absolute. An oxygen product is withdrawn from the column by a pump. A first vaporous nitrogen stream is taken from the top of the higher pressure column, is compressed in a compressor and is used in a gas turbine. Feed to the lower pressure column is derived from a stream of the bottom oxygen-enriched liquid fraction obtained in the higher pressure column. To this end, this stream is subjected to further separation (typically in further rectification column) to form a vaporous nitrogen fraction (a flow of which is condensed and is used as reflux in the lower pressure column) and an oxygen-containing feed to the lower pressure column which flows via an outlet and a condenser to the column. At least 60% of the nitrogen product flowing to the gas turbine is taken from the higher pressure column.