A process for the reduction of the level of water, carbon dioxide and nitrous oxide in a feed air stream prior to cryogenic distillation, the feed air stream comprising from 100 to 1000 ppm CO 2, the process comprising: a) passing said feed air stream at a feed temperature and a feed pressure of from 2 bara to 50 bara in a feed direction through a first adsorbent (34, 34'), which is selected from alumina or impregnated alumina and whose Henry's Law selectivity for CO 2 over N 2 O measured at 30°C is at least 12.5, and subsequently through a second adsorbent (36, 36'), whose Henry's Law constant for the adsorption of CO 2 measured at 30 °C is less than 1020 mmol/g/atm and whose Henry's Law selectivity for CO 2 over N 2 O measured at 30 °C is at most 5; b) ceasing after a first time period to pass said feed air stream to said first (34, 34') and second (36, 36') adsorbents; c) depressurising the gas in contact with the first (34, 34') and second (36, 36') adsorbents to a second pressure lower than the feed pressure and within the range of from 0.5 bara to 2 bara (kPa); d) passing a heated regenerating gas at the second pressure and at a temperature which is between 20 °C and 80 °C to at least the second adsorbent (36, 36') in a direction opposite to the feed direction for a second time period, and subsequently passing a second regenerating gas at the second pressure and at a temperature less than the temperature of the heated regenerating gas to the first (34, 34') and second (36, 36') adsorbents in a direction opposite to the feed direction for a third time period; e) ceasing passing regenerating gas to the first (34, 34') and second (36, 36') adsorbents; f) re-pressurising the gas in contact with the first (34, 34') and second (36, 36') adsorbents to the feed pressure; and g) repeating steps a) to f), wherein the second adsorbent (36, 36') occupies from 25% to 40% by volume of the total volume of the first (34, 34') and second (36, 36') adsorbents, wherein the temperature of the heated regenerating gas is from 10 °C to 60 °C higher than the feed temperature or the temperature of the second regenerating gas, whichever is the higher, wherein the volume of the first adsorbent (34, 34') is selected such that the mass transfer zone for water never extends beyond the boundary between the first and second adsorbent during normal operation, the molar ratio of regenerating gas supplied during step d) to the feed air supplied during step a) is from 0.2 to 0.8, and characterised in that: the first time period is at least 30 min; the method comprises measuring the concentration of CO 2 at the downstream end of the second adsorbent during step (a); and wherein feed air is ceased after the first time period according to step b) when the measured value of CO 2 at the downstream end of the second adsorbent is less than or equal to 100ppb; the second time period is from 2 min to 30 min; the third time period is from 10 to 40 min. a) passing said feed air stream at a feed temperature and a feed pressure of from 2 bara to 50 bara in a feed direction through a first adsorbent (34, 34'), which is selected from alumina or impregnated alumina and whose Henry's Law selectivity for CO 2 over N 2 O measured at 30°C is at least 12.5, and subsequently through a second adsorbent (36, 36'), whose Henry's Law constant for the adsorption of CO 2 measured at 30 °C is less than 1020 mmol/g/atm and whose Henry's Law selectivity for CO 2 over N 2 O measured at 30 °C is at most 5; b) ceasing after a first time period to pass said feed air stream to said first (34, 34') and second (36, 36') adsorbents; c) depressurising the gas in contact with the first (34, 34') and second (36, 36') adsorbents to a second pressure lower than the feed pressure and within the range of from 0.5 bara to 2 bara (kPa); d) passing a heated regenerating gas at the second pressure and at a temperature which is between 20 °C and 80 °C to at least the second adsorbent (36, 36') in a direction opposite to the feed direction for a second time period, and subsequently passing a second regenerating gas at the second pressure and at a temperature less than the temperature of the heated regenerating gas to the first (34, 34') and second (36, 36') adsorbents in a direction opposite to the feed direction for a third time period; e) ceasing passing regenerating gas to the first (34, 34') and second (36, 36') adsorbents; f) re-pressurising the gas in contact with the first (34, 34') and second (36, 36') adsorbents to the feed pressure; and g) repeating steps a) to f), the first time period is at least 30 min; the method comprises measuring the concentration of CO 2 at the downstream end of the second adsorbent during step (a); and wherein feed air is ceased after the first time period according to step b) when the measured value of CO 2 at the downstream end of the second adsorbent is less than or equal to 100ppb; the second time period is from 2 min to 30 min; the third time period is from 10 to 40 min.