Excess of boron in water poses a problem due to adverse effects on crop production as well as human health and aquatic life. This study examined the influence of biofouling of NF and RO membrane on the performance of the membranes in removing boron from a synthetic wastewater effluent. Accelerated laboratory-scale biofouling experiments were carried out with commercial thin film composite NF and RO membranes under controlled conditions. Permeate flux decline, down to less than 25% of its initial value, and substantial decrease in boron rejection were attributed to extensive biofilm growth on the membranes. For the RO membrane, boron rejections declined by 45 and 34% of the initial values for influent boron concentrations of 5.5 and 1.1 mg B/L, respectively, whereas the corresponding declines in boron rejection for the NF membrane were 44 and 13% of the initial values. These adverse effects of biofilm growth on permeate water flux and boron rejection are attributed to both an increase in hydraulic resistance to permeate flow due to bacterial extracellular polymeric substances (EPS) and a biofilm-enhanced concentration polarization near the membrane surface. (c) 2008 Elsevier B.V. All rights reserved.