Abstract: Composite semipermeable membranes are made by reacting a film-forming polymer with a cross-linking monomer to form a polymeric ultrathin film having semipermeable properties. During the formation, a grafting reaction is carried out at reactive sites so there are few remaining active sites where oxidizing attack or fouling can potentially occur. One composite membrane comprises a microporous substrate and an ultrathin polyamide film formed of polyethylenimine cross-linked with a monomer, such as an acyl halide, a chloroformate, an isocyanate or a sulfonyl chloride. Grafted onto the cross-linked polyethylenimine at what would otherwise be secondary amido sites is an epoxide, an aldehyde, an anhydride or a reactive ethylene compound.