A poly(ethylene terephthalate) (PET) membrane was used to study the effect of heavy-ion irradiation on gas permeability and selectivity. In an ambient temperature range of 303-333 K, permeability coefficients for He, N-2, O-2, and CO2 were determined with the vacuum-pressure and time-lag methods. Gas-permeability coefficients of the membranes, which were irradiated with some specific heavy ions, increased remarkably without a significant change in the polymer structure by ion irradiation. The permeation rates of these membranes were inversely proportional to the square root of the molecular weight of the gasses and showed Knudsen flow because of the formation of pores caused by heavy-ion irradiation.