The influence of irradiation and the subsequent etching of latent tracks in poly(4-methylpentene-1) (PMP) films on the transport parameters of the resulting membranes has been studied. The films have been irradiated with accelerated Kr and Xe ions of 4.5 and 1.2 MeV/nucleon in energy at a fluence of 10(6)-10(9) cm(-2). It has been found that the irradiation followed by etching makes it possible to obtain an anisotropic membrane with a nonporous selective layer between two porous layers with tapered pores. The CH4, CO2, and He transport characteristics of the membranes have been examined. It has been shown that these modification methods can significantly increase the gas flux through the membrane. It is believed that the ion track etching procedure as applied to PMP can form the basis for fabricating membranes with a highly permeable, nonporous, gas-selective layer.