In the membrane process industries, membrane cleaning is one of the most important concerns from both economical and scientific points of view. The characterisation of membrane surface by using zeta potential technique provides a more appropriate tool to study the cleaning performance and membrane integrity. The use of zeta potential technique to characterize the cleaning effect on membrane surfaces has very little precedence in literature. In this study the cleaning performance has been studied with membrane zeta potential and hydraulic permeability measurement. Virgin nanofiltration (NF) membranes provided by GE Osmonics (DK, HL and DL), as well as fouled DK membranes were used in this study. Several cleaning agents were investigated, some of them were analytical grade, such as HCl, NaOH and others were commercial grade agents that are already in use in commercial plants. Zeta potential of virgin and fouled NF membranes before and after chemical cleaning were measured and compared. The results of the zeta potential measurements with and without chemical cleaning of virgin and fouled membrane indicated that zeta potential may be used to characterise the membrane surface cleanliness and permeability. The effects of temperature, pH and the cleaning agents on membrane zeta potential were also investigated. The temperature was not found to have any significant effect on membrane zeta potential; however, the effect of pH and cleaning agents had pronounced effects on the surface charge and zeta potential. At high pH values mixed cleaning agents of mixture of trisodium phosphate, sodium tripolyphosphate and EDTA (recommended by membrane manufacturer) had a marked effect on zeta potential of HL and DL membranes compared to other cleaning agents. A DK fouled membrane was found to be almost negatively charged with no isoelectric point compared to a virgin DK membrane. At low pH, the zeta potential values of a fouled DK membrane were in close agreement with those obtained with an untreated virgin membrane after cleaning by NaOH followed by HCl. The zeta potential of all virgin membranes cleaned with SDS changed to more negative for the entire pH range compared to untreated virgin membranes. The study revealed that the SDS cleaning agent had the highest effect on surface charge for both virgin and fouled NF membranes. (c) 2006 Published by Elsevier B.V.