During disinfections, water ultrafiltration membranes are mechanically and chemically stressed. A previous study demonstrated (S. Rouaix, Characterisation and study of ageing of an ultrafiltration membrane, PhD thesis, UniversitΓ© Paul Sabatier, Toulouse, France, 2005) a drop of membranes mechanical properties in contact with bleach accompanied by a degradation of transport properties. It clearly appears fiber embrittlement is a determining criterium, specially for water treaters. It seems thus essential to study long term behavior of polysulfone membranes in focussing on chemical structure-mechanical properties relationships. In this article, polysulfone membranes were immersed in bleach solutions with various pH values. Their structural changes were monitored by mechanical testing, molar mass determination and spectrochemical measurements. The observed degradation, which generates an embrittlement of the fiber, occurs by chain scission (determined by Size Exclusion Chromatography) with the hydroxyl radical OH β’ formed in the bleach solution. The lifetime of the fiber depends on the total chlorine concentration of the ageing solution but also on its pH which drives the formation of hypochlorous acid and hypochlorite ion in great proportion, essential condition for the formation of hydroxyl radicals.