A process for preparing a selective composite membrane, having many practical applications, comprises synthetically impregnating at least some pores, of the upper layer containing the relatively smaller pores, in an asymmetric base membrane, with a filling material comprising an ordered macromolecular structure, by applying under superatmospheric pressure to the asymmetric base membrane, a solution of reactants adapted to interact with the formation of covalent bonds in the ordered macromolecular structure, wherein the dilution of said reactants is sufficiently low, and the pressure is sufficiently high and of such duration, so that formation of covalent bonds is initiated in pores of the upper layer as microreaction vessels and progresses within the pores of the upper layer, the reaction forming such covalent bonds being quenched at any desired stage.