Ion beam irradiation has been widely used to modify the structure and properties of thin polymer surface layers. The transfer of energy from the ions to the polymer chains leads to significant evolution in the chemical structure, microstructure and transport properties of polymeric dense films. This paper focuses on the impact of energetic H+ ions over a range of doses on the transport properties of polysulfone (PSF) asymmetric membranes. There was a large simultaneous decrease in permeance and permselectivity for the asymmetric membranes following H+ irradiation over a range of fluences. These modifications in the transport properties of the asymmetric membranes were not consistent with the results for bulk polymer. At the energies used for this study, the ions penetrated well into the porous substrate of the polysulfone membranes, which caused a collapse of the intermediate porous substrate and formation of a thick non-selective resistance layer. Positron annihilation spectroscopy (PAS) and scanning electron microscopy (SEM) confirmed the existence of a compacted region in porous support.