Abstract: Process for the production of a bilayer membrane consisting of a selectively acting nonporous polymer membrane first layer and a microporous membrane second layer, in which a polymer solution intended for the formation of the first layer is brought into contact with one side of the second layer. A larger amount of polymer solution than is necessary for the formation of the first layer is used, and a portion of the solvent is caused to volatilize through the pores of the second layer. The polymer solution is not caused to enter the pores of the second layer. After the first layer has attained the desired thickness, the excess polymer solution is removed. The first layer may be subjected to a drying and/or cross-linking treatment.