Streaming potential measurements have been used to investigate the surface characteristics of ultrafiltration membranes and to determine the interactions between the foulant (in this study, whey protein concentrate) and the membrane. The zeta potential of several membranes as a function of pH before and after fouling, and after cleaning was determined from these measurements. These results were interpreted in terms of the physicochemical properties of the foulant, membrane surface charge characteristics as a function of pH, and water flux recovery for unused, and after fouled and cleaned membranes. (c) 2005 Elsevier B.V. All rights reserved.