Abstract: Asymmetric gas separation membranes of polymeric materials having selective permeation of at least one gas of a gaseous mixture over that of one or more remaining gases of the gaseous mixture, exhibit improved permeation selectivities for the at least one gas when the asymmetric membranes have been contacted on one or both surfaces with an effective amount of a permeation modifier. The permeation modified asymmetric gas separation membrane having an effective amount of permeation modifier added in a non-homogeneous mode and without loss of Tg.