The effects of membrane fouling on the performance of nanofiltration and reverse osmosis membranes with respect to boron rejection and permeate flux were investigated in this study. A nanofiltration (NF270) membrane and a reverse osmosis (BW30) membrane were used in this investigation. Four typical membrane fouling conditions were simulated under controlled laboratory conditions in a cross-flow membrane system using four model foulants including humic acid, sodium alginate, colloidal silica and CaSO4. Amongst these model foulants, humic acid was found to increase boron rejection whereas the other foulants led to a decrease in boron rejection. Properties of foulants were found to be an important factor that determined the transport of boron through the fouling layer. Results reported in this study also indicate that the extent of flux decline caused by different model foulants differed substantially from one another. The impact of membrane fouling on permeate flux decline was found to be dependent on the initial permeate flux and hydrophobicity of the foulant. On the other hand, membrane scaling was found to be governed by the salt rejection efficiency of the membrane. Cake-enhanced concentration polarisation appears to be a major mechanism that affects boron rejection efficiency of fouled membranes. Crown Copyright (C) 2011 Published by Elsevier B.V. All rights reserved.