an excellent reverse osmosis membrane having a high flux with superior chlorine resistance and low salt passage can be obtained by interfacially condensing a water soluble aromatic polyamide prepolymer with an essentially monomeric , aromatic , amine reactive polyfunctional acyl halide . the polyamide prepolymer may be prepared through the condensation reaction of an aromatic diamine and an aromatic anhydride acyl halide . preferably the amide prepolymer , prepared from metaphenylene diamine and trimelletic anhydride acid chloride , is reacted with trimesoylchloride to form the thin film membrane of the subject invention .