Method for low-temperature air separation, in which an air separation unit (100, 200) having a condenser evaporator and a distillation column system is used, the system having a high-pressure column which is operated at a first pressure range and a low-pressure column which is operated at a second pressure range below the first pressure range, - a first sump liquid, which has a higher oxygen content and a lower nitrogen content than atmospheric air, and a first top gas, which has a lower oxygen content and a higher nitrogen content than atmospheric air, being formed in the high-pressure column by low-temperature rectification, - a second sump liquid, which has a higher oxygen content and a lower nitrogen content than the first sump liquid, and a second top gas, which has a higher nitrogen content and a lower oxygen content than the first sump liquid, being formed in the low-pressure column by low-temperature rectification, - a portion of compressed and cooled air being guided through the condenser evaporator, liquefied or partly liquefied therein and fed into the distillation column system, a liquid having an inlet oxygen content of 15% to 45%, in particular of 20% to 40%, being evaporated or partly evaporated in the condenser evaporator, and - a further portion of the compressed and cooled air is fed into the distillation column system, without being passed through the condenser evaporator, characterized in that - the second top gas or a portion of the second top gas is taken from the low-pressure column as impure nitrogen with an oxygen content of 8 to 15 mole percent, and - a portion of the impure nitrogen is successively heated as a return quantity, compressed to a pressure within the first pressure range, cooled and fed into the high-pressure column. - a first sump liquid, which has a higher oxygen content and a lower nitrogen content than atmospheric air, and a first top gas, which has a lower oxygen content and a higher nitrogen content than atmospheric air, being formed in the high-pressure column by low-temperature rectification, - a second sump liquid, which has a higher oxygen content and a lower nitrogen content than the first sump liquid, and a second top gas, which has a higher nitrogen content and a lower oxygen content than the first sump liquid, being formed in the low-pressure column by low-temperature rectification, - a portion of compressed and cooled air being guided through the condenser evaporator, liquefied or partly liquefied therein and fed into the distillation column system, a liquid having an inlet oxygen content of 15% to 45%, in particular of 20% to 40%, being evaporated or partly evaporated in the condenser evaporator, and - a further portion of the compressed and cooled air is fed into the distillation column system, without being passed through the condenser evaporator, - the second top gas or a portion of the second top gas is taken from the low-pressure column as impure nitrogen with an oxygen content of 8 to 15 mole percent, and - a portion of the impure nitrogen is successively heated as a return quantity, compressed to a pressure within the first pressure range, cooled and fed into the high-pressure column.