The present work addresses the mechanistic aspect of hypochlorite attack on a commercial polyamide membrane and the oxidative degradation taking place under active (applied pressure with constant stirring) and passive (unpressurised and unstirred) conditions. The degree of membrane degradation was enumerated in terms of the decline in membrane performance, calculated from permeability and salt rejection for reverse osmosis (RO) feed water. For active exposures both permeability and salt rejection changed significantly whilst only permeability varied for passive exposure. The structural changes in the polymeric backbone of the active surface layer were studied with attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR). For the chlorine damaged membranes, distinct peak suppression has been observed around 3330 cm(-1). Minor but significant shift in amide I and II bands and peak suppression in the ring stretching vibration at 1609 cm(-1) was also noticed. The suppression or shift in the characteristic peaks was observed for both active and passive contact, however the extent differed. The progress of membrane damage has been confirmed to advance up to ring chlorination, however the N-chlorinated substituent may not be considered as a short-lived intermediate in view of the fact that the chlorine substitution on nitrogen has been indicated through a change in the amide 11 band in the final product. Perhaps the reaction products are both N-chlorinated products and ring chlorinated compounds, most likely their existence being influenced by operating parameters. Especially pressure seems to have effect on the chlorine attack of polyamide surface, seeing that the rearrangement of N-chlorinated substituent to ring chlorination is favored by pressurized environment. Formation of ring chlorinated products appears to be the destroying phenomenon for the polyamide surface resulting in salt passage. Chlorine damaged membranes were tested for Fujiwara test, which is an indicative test generally performed in membrane autopsies to check the degradation of polyamide membranes with halogens. Whilst the membrane degradation was evident from the salt rejection values and the FT-IR patterns, Fujiwara analysis failed to diagnose the chlorine exposure at low concentrations, especially under passive conditions. (C) 2009 Elsevier B.V. All rights reserved.