In this study, reverse osmosis (RO) composite membrane with extra-thin separation layer was prepared by the interfacial polymerization (IP) of metaphenylene diamine (MPD) with trimesoyl chloride (TMC) on the surface of polysulfone (PS) support membrane. The properties and structures of skin layer of RO composite membranes were characterized by FTIR and SEM, it was found that IP had occurred and the separation layer was formed. The effects of the monomer concentration on membrane flux and salt rejection were investigated, and the optimum concentration of MPD and TMC were 2 and 03% (w/v), respectively. To improve flux, the phase-transfer catalyst was added to the water phase, and the effects were remarkable when the concentration of MPD was low, in which both salt rejection and flux increased by 20% than initial results. When some of the hydrophilic additives Such as, alcohols and phenols were added into water phase, the flux of the prepared membrane increased from 13.03 to 33.42 L/(m(2) h) without loss in salt rejection. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 112: 2066-2072, 2009