The permeability of a composite membrane consisting of a homogeneous layer and a porous layer has been derived theoretically by assuming that the permeation through the homogeneous layer obeys Fick's law and that permeation through the porous layer is free molecular flow. The activation energy of the flow is described by three-dimensionless parameters, ϕ = |P12/|P2, θ = d1/d2, and σ = |P1/|P2. |P12, |P1, and |P2 are the permeability coefficients of the composite membrane, the homogeneous layer and the porous layer, respectively, d1 and d2 are the thickness of the two layers. Once these parameters are determined, information on the structure of the membrane can be obtained (i.e., the pore size and the pore density). The permeabilities of various gases through homogeneous polycarbonate membrane, neutron-irradiated, nonsodium hydroxide-etched polycarbonate membrane, and their composite membrane were tested. A two-layer series model, incorporating the effect of neutron irradiation which produces some nonpenetrating pores in the porous membrane layer, is proposed and agreed quite well with the experimental data.