A novel route to the preparation of nanofiltration membranes has been proposed. The method consists of the photografting of polymer on to the surface of porous membranes. The method was tested with polysulfone ultrafiltration membranes and poly(acrylic acid) in water solution. It has been shown that the surface modification is stabilised by crosslinking with N,N'-methylene bis acrylamide and that the grafting does not require any photoinitiator and any purification of the reactants. Moreover, by the proper choice of the wavelengths, the UV irradiation does not induce any significant copolymerisation of the reactants inside the aqueous solution. The irradiation conditions, the monomer concentration and the permeability of the support membrane were examined. The water softening properties of the novel membranes were measured in dead-end mode and at a 95% recovery. A large range of performances has been obtained which demonstrates the feasibility of the UV irradiation as an environmentally friendly and versatile (flexible) technology to prepare nanofiltration membranes.