Process for the low-temperature separation of air, in which an air separation system having a main heat exchanger and a distillation column system (6, 7) is used, which distillation column system comprises a high-pressure column operated at a first pressure level, a low-pressure column operated at a second, lower pressure level, and a mixing column, and in which - an oxygen-rich stream (n) having a first oxygen content is withdrawn in the liquid state from the low-pressure column and is fed in the liquid state with the first oxygen content into the mixing column, - in addition, a first compressed-air stream (h) is fed in the gaseous state into the mixing column and, in the mixing column, is sent in counterflow to the oxygen-rich stream (n) having the first oxygen content, - an oxygen-rich stream (o) having a second oxygen content below the first oxygen content is withdrawn from the mixing column overhead and is passed out of the air separation system, - the first compressed-air stream (h) is formed using air that is compressed to a starting pressure level above the first pressure level and thereafter is cooled to a first temperature level, is fed at the first temperature level to a first turbine and is expanded in the first turbine, - a liquid oxygen-rich air product is passed in the liquid state out of the air separation system at least at times, - a second compressed-air stream (g) is fed into the high-pressure column, which second compressed-air stream is likewise formed using the air that is compressed to the starting pressure level and thereafter cooled to the first temperature level and expanded in the first turbine, - a third compressed-air stream (f) is fed into the low-pressure column, which third compressed-air stream is formed using air that is compressed to the starting pressure level and thereafter cooled to a second temperature level, fed at the second temperature level to a second turbine, expanded in the second turbine and cooled further in the main heat exchanger to a third temperature level, - the air is expanded in the first turbine to the first pressure level and in the second turbine to the second pressure level, and the mixing column is operated at the first pressure level or a third pressure level that differs from the first pressure level by at most 1 bar, - the first temperature level is at least 20 K below the second temperature level, and - the process comprises a first process mode and a second process mode, wherein, in the first process mode, the liquid oxygen-rich air product is passed out of the air separation system in a larger amount than in the second process mode, and in the first process mode, a larger amount of air is expanded in the second turbine than in the second process mode and as a result, at the same time the third compressed-air stream (f) in the first process mode comprises the same larger amount of air than in the second process mode. - an oxygen-rich stream (n) having a first oxygen content is withdrawn in the liquid state from the low-pressure column and is fed in the liquid state with the first oxygen content into the mixing column, - in addition, a first compressed-air stream (h) is fed in the gaseous state into the mixing column and, in the mixing column, is sent in counterflow to the oxygen-rich stream (n) having the first oxygen content, - an oxygen-rich stream (o) having a second oxygen content below the first oxygen content is withdrawn from the mixing column overhead and is passed out of the air separation system, - the first compressed-air stream (h) is formed using air that is compressed to a starting pressure level above the first pressure level and thereafter is cooled to a first temperature level, is fed at the first temperature level to a first turbine and is expanded in the first turbine, - a liquid oxygen-rich air product is passed in the liquid state out of the air separation system at least at times, - a second compressed-air stream (g) is fed into the high-pressure column, which second compressed-air stream is likewise formed using the air that is compressed to the starting pressure level and thereafter cooled to the first temperature level and expanded in the first turbine, - a third compressed-air stream (f) is fed into the low-pressure column, which third compressed-air stream is formed using air that is compressed to the starting pressure level and thereafter cooled to a second temperature level, fed at the second temperature level to a second turbine, expanded in the second turbine and cooled further in the main heat exchanger to a third temperature level, - the air is expanded in the first turbine to the first pressure level and in the second turbine to the second pressure level, and the mixing column is operated at the first pressure level or a third pressure level that differs from the first pressure level by at most 1 bar, - the first temperature level is at least 20 K below the second temperature level, and - the process comprises a first process mode and a second process mode, wherein, in the first process mode, the liquid oxygen-rich air product is passed out of the air separation system in a larger amount than in the second process mode, and in the first process mode, a larger amount of air is expanded in the second turbine than in the second process mode and as a result, at the same time the third compressed-air stream (f) in the first process mode comprises the same larger amount of air than in the second process mode.