The objective of this study was to investigate the contribution of electrostatic interactions to the rejection of selected organic acids by nanofiltration membranes. Effective membrane surface charge (determined by two methods of zeta potential measurements) of two commercial nanofiltration membranes was examined at different pH values and feed water chemistries. Six different organic acids were selected for this study representing typical physico-chemical properties of emerging organic trace pollutants. Findings of this study indicated that the rejection of negatively charged organic acids was larger than expected based on steric exclusion and was primarily driven by the surface charge of the membrane and correlated with the degree of ionization of the solute. Increasing feed water pH resulted in an increased negative surface charge, an increased percentage of solutes in the deprotonated state and an increased rejection through electrostatic repulsion. The rejection of the pharmaceutical residue ibuprofen, an organic acid with hydrophobic properties, was also pH dependent, but at pH values below the pK(a) ibuprofen adsorbed and partitioned through the membrane. At pH above the pK(a), adsorption of ibuprofen was minimal due to an increased solubility and the dominant role of electrostatic repulsion. The presence of calcium in the feed water lowered the effective mernbrane surface charge (as determined by electrophoretic mobility measurements) of both membranes tested, however, rejection of negatively charged organic solutes decreased only for membranes with a molecular weight cut-off larger than the solute molecular weight. (C) 2004 Elsevier B.V. All rights reserved.