Nanofiltration is an attractive technology for the production of drinking water from surface water. However, membrane fouling, mainly caused by natural organic matter, increases the costs and limits the water recovery. In this study, it was investigated whether the decomposition of natural organic matter in the concentrate stream by O(3), has a positive effect on the membrane flux of four nanofiltration membranes (NF 90, NF 270, Desal 51, NF-PES 10). The results show that O(3) oxidation causes a significant alleviation of membrane fouling for all investigated membranes. This is caused by the selective removal of unsaturated bonds and hydrophobic components in the dissolved organic matter, and by the decomposition of molecular chains into smaller fragments by O(3). However, the chemical oxygen demand could not be reduced by more than 20%. The performance of O(3) + H(2)O(2) was only slightly superior to O(3) alone, using an identical O(3) dose. (C) 2011 Elsevier B.V. All rights reserved.