AbstractIn the present case, Polyethylene terephthalate (PET) membranes of thickness 25 μm were irradiated by 100 MeV Cl9+ ion beam with ion fluence of 107 ions/cm2. These membranes were etched in 6N NaOH at 60 °C at different etching times to generate pores. The permeabilities of nitrogen, oxygen, methane, carbon dioxide as well as hydrogen were measured for etched membranes at various etching times. The permeability of gases was found to increase with etching time and a rapid increase was noticed after a critical etching time when the etched tracks meet at their vertexes. The selectivity of hydrogen gas over other gases was also observed.