The growing demand on water resources has increased interest in wastewater reclamation for potable reuse, in which rejection of organic micropollutants such as disinfection by-products (DBPs), endocrine disrupting compounds (EDCs), and pharmaceutically active compounds (PhACs) is of great concern. The objective of this study was to investigate the rejection of DBPs, EDCs, and PhACs by nanofiltration (NF) and reverse osmosis (RO) membranes as a function of their physico-chemical properties and initial feed water concentration. Experimental results indicated that negatively charged compounds could be rejected very effectively (i.e., >90%) regardless of other physico-chemical properties of the tested compounds due to electrostatic exclusion. No time-dependency was observed for rejection of charged compounds. In contrast, rejection of non-charged compounds was generally lower (<90% except for one case) and influenced mainly by the molecular size of the compounds. A clear time-dependency was observed for rejection of non-charged compounds, attributable to compound adsorption on the membrane. It was demonstrated that feed water concentration influenced rejection efficiency of the membrane. Experiments conducted at a low ng/l concentration range resulted in lower rejection efficiency as compared to experiments conducted at a mug/l range, suggesting the need to conduct experiments at the relevant concentration of interest. (C) 2003 Elsevier B.V. All rights reserved.