The possibility of concentrating a trace component by pressure swing adsorption is examined both theoretically and experimentally. During the blowdown of an equilibrated adsorption column the lighter component is desorbed first so that the adsorbed phase becomes progressively enriched in the more strongly adsorbed species. If the selectivity is high, most of the lighter component can be removed by atmospheric blowdown and the more strongly adsorbed species may then be recovered, in concentrated form, by vacuum desorption. The practical feasibility of a cryogenic PSA process as a means of concentrating traces of hydrogen from a helium carrier is demonstrated. Hydrogen may be recovered at greater than 90% purity and approximately 80% recovery from a feed containing less than 0:2% hydrogen in helium.