Separation of multicomponent gas mixtures by pressure swing adsorption. The multicomponent mixtures contain a primary and a secondary key component and a tertiary component which is present in a concentration less than either the primary or secondary key component and which is more strongly sorbed than both these key components. The selective adiabatic pressure swing cyclic adsorption system, for the separate recovery of high purity primary and secondary key components, comprises different steps applicable to the various adsorption trains, each containing a vessel A and a vessel B. These different steps are: (1) adsorption (both vessels A and B); (2) high pressure rinse (vessel B), pressure equalization and repressurization (vessel A); (3) depressurization (vessel B), purge (vessel A); (4) evacuation (vessel B), optional purge (vessel A); (5) repressurization (vessel B), pressur equalization and repressurization (vessel A). An example is the treatment of effluent gas from a hydrodesulphurization plant after removal of the sulphur, wherein hydrogen (primary key component) and methane (secondary key component) are separately recovered and substantially freed of C2 + hydrocarbons (tertiary component) present in such effluent gas.