Semipermeable composite membranes which comprise an interpenetrating polymer network of the isocyanate-capped polymer which is the reaction product of a polyether and an isocyanate with a copolymer containing at least two saturated or unsaturated heterocyclic nitrogen-containing compounds composited on a porous backing support material may be used in various separation systems. These separation systems may include reverse osmosis, ultrafiltration, gas separation, dialysis, etc., an especially useful system being the separation of sugars.