The efficiency of the separation of some inert gases (helium, nitrogen, argon, krypton, xenon) and hydrogen on nuclear track membranes of polyethylene terephthalate (PET) with pores of size 2-10 nm have been investigated. The membranes were prepared either by the freeze-drying method or by partial contraction of the porous structure. Membranes with pores over 6 nm in diameter are found to have an ideal gas separation coefficient, close to the theoretical (Knudsen) one, under normal conditions. A gradual decrease in the size of the pores leads to an increase in the reduced ideal separation coefficient of the hydrogen-helium pair for membranes obtained by the freeze-drying method, and in that of the helium-argon and helium-nitrogen pairs for membranes obtained by partial "healing" of pores. For other gas pairs a decrease in the separation coefficients was observed. A theoretical model is proposed that allows description of the observed regularities on the basis of the surface diffusion of components, taking into account the structural peculiarities of the membranes and the isothermal entrainment effects between free and adsorbed gas molecules.