The invention provides a process for improving the antifouling properties and for increasing the selectivity of commercial composite polyamide nanofiltration (NF) and reverse osmosis (RO) membranes comprising circulating a low concentration of water soluble monomers and an initiating redox couple in an aqueous solvent, without transverse pressure over the surface of the membrane thereby effecting a free-radical graft polymerization on the surface of the membrane.