Membranes prepared by extensive grafting of acrylamide onto nylon-6 films exhibit high water sorption capacity. Swelling of the membranes increases after treatment with aqueous solutions of formic acid. Sorption of water into these annealed membranes reaches values of ca. 25 moles of water per mole of grafted acrylamide. The sorption characteristics of the swollen membrane-gels crosslinked with bis-acrylamide were explored. Membranes grafted with acrylamide, crosslinked with bis-acrylamide and annealed with formic acid reach the maximum swelling capacity at low graft yields.