The advantages of usmg asymmetrrc polymeric membranes with a dense skm and a mlcroporous backmg for separation of permanent gas muttures have been pomted out A gross picture of the "pore size" varratlon from the skm top through the transltlon layer to the top of the mlcroporous backmg has been proposed Gas species are transported through the top skm layer and the transItion region actmg as a dense film by F~ckmn dlffuslon Knudsen dBuslon IS assumed to control gas transport through the mlcroporous backmg The separation factor for an asymmetrzc dense membrane of the above type IS found to vary between the higher values of selective gas permeation through a dense film and the lower values of gaseous dtiuslon separation through a mlcroporous film Several vmations of the basic model are possible depending on the nature of the Knudsen flux expresslon through a mlcroporous media, the nature of the "pore size" vmation with distance from the skm top and the variation of the "operational skm tickness" with the gas species Models havmg one unknown constant predict separation factors or permeation ratios of two permanent gases m reasonable agreement with Gantzel and Merten's data on gas permeation through Loeb-Sounralan type CA membranes Predicted permeabdltles have poor agreement with data when the "operational skin thickness" IS independent of the gas species