High flux semipermeable membranes which comprises the polymerized reaction product within and/or on a porous support backing material may be prepared by contacting the porous support such as a polysulfone with an aqueous solution of a polyamine which may, if so desired, contain a polar aprotic solvent not reactive with the amines, a polyhydric compound and an acid acceptor. The surface of the coated support is freed of excess solution and thereafter contacted with an organic solution of a polyacyl halide for a period of time sufficient to form a polymerized reaction product within and/or on the support material. The resulting composite is then treated with a hydroxy polycarboxylic acid, polyaminoalkylene polycarboxylic acid, suffonic acid, amine salts of acids, amino acid, amino acid salt, polymeric acid and inorganic acid, before drying of the membrane. This will enable the membrane to be stored in a dry manner prior to use thereof in a separation process. The resultant membrane composite may be used in separation processes such as desalination of brackish or sea water or the softening of domestic hard water.