A method for determining conditions for the reduction, prior to cryogenic distillation, of the level of water, carbon dioxide and nitrous oxide in a feed air stream in a process comprising: (a) passing said feed air stream at a feed pressure in a feed direction through a first adsorbent which has a Henry's Law selectivity for carbon dioxide over nitrous oxide measured at 30 °C of at least 12.5, and subsequently through a second adsorbent whose Henry's Law constant for the adsorption of N 2 O measured at 30 °C is less than 500 mmol/g/atm and whose Henry's Law selectivity for CO 2 over N 2 O is at most 5; (b) ceasing to pass said feed air stream to said first and second adsorbents after a first time period; (c) depressurising the gas in contact with the first and second adsorbents to a second pressure less than the feed pressure; (d) passing regenerating gas at the second pressure and at a first temperature which is in the range 140 to 220 °C to at least the second adsorbent in a direction opposite to the feed direction for a second time period, and subsequently passing the regenerating gas at the second pressure and a second temperature less than the first temperature to the second and first adsorbents in a direction opposite to the feed direction for a third time period; (e) ceasing passing regenerating gas to the first and second adsorbents; (f) repressurising the gas in contact with the first and second adsorbents to the feed pressure; (g) repeating steps (a) to (f); wherein the first adsorbent occupies from 25% to 40% by volume of the total volume occupied by the first and second adsorbents; and wherein the molar ratio of the regenerating gas to the feed air supplied during each iteration of the steps (a) to (f) is from 0.08 to 0.5; which method comprises: (i) placing an analyser for CO 2 concentration measurement at a chosen position within the length of the second adsorbent; (ii) determining the threshold concentration measured by the CO 2 analyser when a desired maximum level of N 2 O is simultaneously obtained at the downstream end of the second adsorbent in the feed direction; (iii) using the result of step (ii) to determine the maximum duration of the first time period, such that the maximum duration of the first time period is the time taken from commencing passing the feed air stream to the first and second adsorbents to the measurement by the analyser of the determined threshold concentration of CO 2. (a) passing said feed air stream at a feed pressure in a feed direction through a first adsorbent which has a Henry's Law selectivity for carbon dioxide over nitrous oxide measured at 30 °C of at least 12.5, and subsequently through a second adsorbent whose Henry's Law constant for the adsorption of N 2 O measured at 30 °C is less than 500 mmol/g/atm and whose Henry's Law selectivity for CO 2 over N 2 O is at most 5; (b) ceasing to pass said feed air stream to said first and second adsorbents after a first time period; (c) depressurising the gas in contact with the first and second adsorbents to a second pressure less than the feed pressure; (d) passing regenerating gas at the second pressure and at a first temperature which is in the range 140 to 220 °C to at least the second adsorbent in a direction opposite to the feed direction for a second time period, and subsequently passing the regenerating gas at the second pressure and a second temperature less than the first temperature to the second and first adsorbents in a direction opposite to the feed direction for a third time period; (e) ceasing passing regenerating gas to the first and second adsorbents; (f) repressurising the gas in contact with the first and second adsorbents to the feed pressure; (g) repeating steps (a) to (f); (i) placing an analyser for CO 2 concentration measurement at a chosen position within the length of the second adsorbent; (ii) determining the threshold concentration measured by the CO 2 analyser when a desired maximum level of N 2 O is simultaneously obtained at the downstream end of the second adsorbent in the feed direction; (iii) using the result of step (ii) to determine the maximum duration of the first time period, such that the maximum duration of the first time period is the time taken from commencing passing the feed air stream to the first and second adsorbents to the measurement by the analyser of the determined threshold concentration of CO 2.