A method of casting ultra-thin semipermeable membranes comprises the steps of: (A) applying a coating of a film forming water soluble solution, containing a leachable pore producing agent and a polymer which is not substantially degraded by organic polar solvents, onto a smooth casting surface, (B) drying the coating to form a support film about 0.5-10 mils thick, (C) applying a coating of an organic polymeric membrane coating solution onto the support film, and drying the coating to form a semipermeable reverse osmosis membrane film about 0.01-1.0 mil thick and (D) applying a leaching liquid to the films, to cause pore formation in the support film and to form a composite consisting of a 0.01-1.0 mil thick semipermeable membrane, supported by a 0.5-10 mils thick water soluble film support of between about 5-50% porosity.