The concentration of adsorbate in the feed gas to an on-stream bed of a cyclical swing adsorption process is monitored and the data processed to predict the time required to complete the on-stream mode of that bed and the purge flow rate and/or other regeneration mode operating condition of the concurrently off-stream bed is modified in response to changes in said predicted time whereby the regeneration mode of the off-steam bed is completed at the same time as the on-stream mode of the concurrent on-stream bed.