A novel and general method for modifying hydrophobic poly(arylsulfone) ultrafiltration membranes to produce highly hydrophilic surfaces has been developed. This method consists of the direct UV irradiation of poly(arylsulfone) membranes in the presence of water or methanol soluble monomers. It was discovered that the poly( arylsulfone)s are intrinsically photoactive and that no photoinitiators are required for this process. Membranes were obtained in which polymeric segments derived from the hydrophilic vinyl monomers are directly bound to the poly(arylsulfone) chains by direct chemical bonds. A mechanism involving a photochemically induced free radical cleavage of the poly(arylsulfone) chains has been proposed. The effects of the irradiation conditions and the monomer concentration of three monomers : 2-hydroxyethyl methacrylate (HEMA), glycidyl methacrylate (GMA) and methacrylic acid (MAc) on the extent and depth of modification in the membrane were examined.