Ion beam irradiation was used to modify the surface of a sulfonated polysulfone water treatment membrane. A beam of 25 keV H+ ions with four irradiation fluences (1 x 10(13), 5 x 10(13), 1 x 10(11), and 5 x 10(14) ions/cm(2)) was used to study the effects of ion beam irradiation on chemical structure, surface morphology, rnicrostructure, and performance. XPS and ATR-FTIR analyses were performed on the virgin and irradiated membranes in order to determine the changes to chemical structure incurred by ion beam. irradiation. The results showed that some sulfonic and C-H bonds were broken and new C-S bonds were formed after irradiation. AFM analysis showed that the roughness of the membranes decreased after irradiation, and the decrease in surface roughness was proportional to the increase in irradiation fluence. An increase in flux after ion beam irradiation was also observed along with a smaller flux decline during operation. Flux was not a function of irradiation fluence. Hydrophobicity, pore size distribution, and membrane rejection efficiencies were not affected by ion beam irradiation. Overall, irradiation led to an improvement in membrane performance.