- Polymer membranes are often used in water treatment industry, as a reliable economic solution for purification of surface water streams. Among a variety of membrane processes the ultrafiltration (UF) membranes combine high pathogen removal ability with minimal energy consumption. Since the membranes are considered to be chemically and mechanically stable, in-place cleaning is routinely performed with strong oxidizing agents, such as hypochlorite. The results of the current study, however, clearly indicate that the mechanical strength of UF membranes deteriorates upon hypochlorite cleaning. Values of ultimate tensile strength, ultimate elongation and Young’s modulus decreased as the dose (concentration × contact time) of hypochlorite was increased, with the decreases being more marked for cellulose acetate membranes than for polyethersulfone membranes. Scanning probe microscopy showed membrane aging in response to hypochlorite treatment. The aging was related to gradual chain breaking in membrane skin layer, as reflected by the disappearance of O–C–O and C–S absorbance peaks in ATR-FTIR spectra of treated cellulose acetate and polyethersulfone membranes, respectively. Since the degree of scission was linearly related to the deterioration in ultimate tensile strength, we here propose that ultimate tensile strength measurements can be used as a simple macroscopic test for periodic inspection of the degree of degradation of UF membranes exposed to in-place chemical cleaning.