Abstract: For producing ultrapure nitrogen by low-temperature fractionation of air in a rectification system having at least one rectification column, compressed and purified feed air is conducted into a high pressure column, an oxygen-free pressurized nitrogen fraction is produced from the high pressure column, conducted into a low-pressure column and in the low-pressure column carbon-monoxide-free low-pressure nitrogen is produced as overhead gas. The oxygen-free pressurized nitrogen fraction (1) is either taken off in the liquid state from an upper region of the high pressure column or is provided from a liquid tank (22) containing stored oxygen-free nitrogen and the oxygen-free pressurized nitrogen fraction (1) is expanded into the bottom-heated low-pressure column (2).