a composite membrane comprises a substrate made from a polymer selected from copolymers and homopolymers of ethylenically unsaturated nitriles , which substrate has preferably first been treated with a pore protector in absence of curing agents and catalysts therefor , prior to applying as the final coating a silicone layer , which is crosslinked . the pore protector , which may be , for example , a hydroxy - terminated polysiloxane , particularly a silanol - terminated polysiloxane , serves the dual purpose of preventing the pores from collapsing , when the support is dried during the curing of the silicone layer , and of preventing passage of the coating material deeply into the pores and thus also preventing an undue reduction of the flux of the finished coated membrane . such composite membranes include solvent stable membranes which swell to an extent of no more than about 10 % when immersed in various organic solvents , and their mixtures with each other and / or water .