Abstract: A method of producing a membrane comprises providing a solution containing a plurality of polymers in a non-aqueous liquid solvent, at least one of the polymers (hereinafter referred to as the "first polymer") having been prepared by polymerizing at least one vinyl or acrylic monomer, at least one other of the polymers being a membrane-forming hydrophobic polymer (hereinafter referred to as the "second polymer"), the first polymer and the second polymer being mutually incompatible in said solution, and contacting the solution with an aqueous medium thereby to precipitate the polymers from the solution to form the membrane.