Separation of multicomponent gas mixtures containing a primary key component, a secondary key component that is more strongly sorbed and a minor quantity of one or more tertiary components which are less strongly sorbed than the secondary key component but more strongly sorbed than the primary key component. The mixtures are subject to selective adsorption in an adiabatic pressure swing cyclic system for the separate recovery of high purity primary and secondary key component. The system comprises different steps applicable to the various adsorption trains, each containing a vessel A and a vessel B. The different steps are: (1) Adsorption (vessels A and B). (2) High pressure rinse (vessel A); two pressure equalization steps and desorption (vessel B). (3) Desorption (vessel A), low pressure purge and pressurization (vessel B). (4) Desorption (vessel A), adsorption (vessel B).; (5) Evacuation (vessel A), two pressure equalization steps and desorption (vessel B). (6) Pressure equalization (vessel A), low pressure purge (vessel B). (7) Pressurization (vessels A and B). An example is the treatment of a Shift converter effluent gas from a hydrocarbon reformer plant, wherein hydrogen (primary key component) and carbon dioxide (secondary key component) are separately recovered substantially freed of minor components such as methane, carbon monoxide and nitrogen (tertiary components).