The gas permeability of O-2 and N-2 for homogeneous and composite membranes prepared from poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) samples with different molecular weight parameters was investigated. Temperature dependencies of gas permeability coefficients and permselectivity were determined for homogeneous membranes. It was established that gas permeability coefficients of homogeneous membranes depend on molecular weight of the polymers used. The gas permeability of composite membranes with a PPO selective layer was investigated as a function of PPO intrinsic viscosity [eta] and its casting solution concentration (c). It was shown that under the condition [eta]. c = const it is possible to obtain composite membranes with the same transport properties by using polymers with different molecular weight parameters.