Abstract: The invention provides a membrane for separating molecules from mixtures, which membrane consists virtually exclusively of molecular sieve crystals, has a thickness of 0.01-10 mm and has a separating top layer with a thickness of 0.01-10 .mu.m and an effective pore size of less than 1.5 nm, which pore size can be adjusted depending on the molecules to be separated off. The separating membrane can be produced by a method in which: a) pulverulent molecular sieve material and oxides constituting the molecular sieve material, especially zeolite and kaolin and/or silicon dioxide, are mixed, preferably in the presence of water; b) the mixture is shaped; c) the moulding is calcined; d) the calcined moulding is treated with an aqueous solution which promotes crystallization; and e) the moulding is treated with a solution of the oxides constituting the molecular sieve material, and then rinsed and dried. The separating membrane can consist of one component or be combined with, for example, an alumina support.