Reverse osmosis membranes based on partially disulfonated copolymers of biphenol and aryl sulfone (BPS) have been studied in an asymmetric architecture. In these films, BPS represents both the porous support and solute rejection layer of the membrane. A procedure was developed to fabricate these asymmetric films through phase inversion of BPS solutions. Recipe optimization and the inclusion of several post-treatment steps to density the rejection layer and heal defects gave the largest improvements in salt passage and are described in detail. A strong dependence of the final salt passage on ionic strength and temperature of the annealing solution is demonstrated. Under 2000 ppm NaCl and 15.5 bar test conditions, membranes were developed with salt passage from 0.34% to 1.81% across a flux range of 2.5-20 lmh. This performance represents a step change in the capability of BPS membranes for reverse osmosis. To achieve competitiveness with commercial polyamide membranes, several challenges required for continued improvements and commercialization are discussed. (C) 2013 Published by Elsevier B.V.