The modifications in poly (ethersulfone) FES membranes induced by swift heavy ion irradiation are analyzed. PES membrane of 25 mu m was irradiated by Cl(9+) ions of 120 MeV at IUAC New Delhi, India with ion fluence of 10(6)-10(8) ion/cm(2). The changes due to exposure to high-energy ions were investigated by Dynamic mechanical analysis and gas permeation. The strength of the polymer membranes increases with increasing ion fluence. Gas permeation through these membranes before and after irradiation using hydrogen and carbon dioxide gases shows that permeability for both gases increased after irradiation but permeability for hydrogen is higher than carbon dioxide due to its small molecular size. This is due to the larger electronic energy loss by Cl(+9) ions creating large damage area, which is etched rapidly and large permeability is observed in the less etching time. The gas permeability of both the gases are almost found same from both the side of the membrane consequently, the shape of the tracks is almost cylindrical. (C) 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.