Abstract: Glassy-type gas separating membranes are conditioned to a state of enhanced permeability through controlled sorption of a selected conditioning gas such as CO.sub.2. Membranes are swelled to a state of plasticity without being rubberized and maintained in this state to allow their permeabilities to settle to a new, higher steady-state value. Upon depressurization, conditioned membranes exhibit enhanced permeability over untreated membranes, without serious loss of permselectivity. Membranes so conditioned must be maintained with a minimum sorption level of a conditioning or maintaining gas pending use to retain their improved characteristics.