Selected aromatic amides were used to model the chemical reactivity of aromatic polyamides found in thin-film composite reverse osmosis (RO) membranes. Chlorination and possible amide bond cleavage of aromatic amides upon exposure to aqueous chlorine, which can lead to membrane failure, were investigated. Correlations are made of the available chlorine concentration, pH, and exposure time with chemical changes in the model compounds. From the observed reactivity trends, insights are obtained into the mechanism of RO membrane performance loss upon chlorine exposure. Two chemical pathways for degradation are shown, one at constant pH and another that is pH-history dependent. An alternative strategy is presented for the design of chlorine-resistant RO membranes, and an initial performance study of RO membranes incorporating this strategy is reported. (C) 2003 Wiley Periodicals, Inc.