In water treatment processes, membrane cleaning is of paramount economic and scientific importance. Traditionally, ultrafiltration membranes fouled with organic matter are cleaned with hypochlorite, an operation that usually results in complete restoration of the initial flux. Although the cleaned membranes are deemed suitable for continued operation, membrane integrity may have been damaged (manifested as holes in the membrane skin) by the cleaning process. To assess the degree of chemical resistance of polyethersulfone (PES)-based membranes, the membranes were fouled with bovine serum albumin and cleaned with various concentrations of sodium hypochlorite. The properties of pristine, fouled, and cleaned membranes were compared by spectrochemical analysis, mechanical testing, microscopy imaging, and streaming potential and permeability measurements. Our results suggest that cleaning does have a major impact on the performance and properties of PES membranes. Effective removal of the foulant from the membrane surface resulted in more severe fouling and increased electronegativity of the cleaned membrane. The increased electronegativity was related to chain scission of the polymer, leading to the formation of phenyl sulfonate. The deterioration in the mechanical strength of hypochlorite-treated membranes indicated a loss of membrane integrity. The transmembrane streaming potential technique provided information on membrane pore size, cleaning efficiency, and membrane integrity. (c) 2007 Elsevier B.V. All rights reserved.