Irradiation is an important tool to modify the polymeric material. For the present study a large membrane roll of PET was irradiated by Cl-35(9+) ion of 120 MeV, and etched chemically in 6 N NaOH at 60 +/- 2 degrees C. The membrane roll was characterized by hydrogen and carbon dioxide gas permeation. The permeability increases with the etching time and at a particular etching time the permeability increases rapidly for both gases. The etching time, at which a rapid increment was observed, remains almost the same for the entire length of the roll horizontally, but different in vertical direction. The entire length of the roll was also characterized by the current monitored etching, which showed changes in the etching time for the middle and side parts of the film roll. Ion permeation measurements show that pores in a polymeric membrane are charged or neutralized, which depends upon the variation in the concentration of the solvent. The negative charge on the surface of the pores is also responsible for their cation selectivity.