BACKGROUND: Membrane fouling by humic acids limits the water recovery of nanofiltration in drinking water production. This article investigates if membrane fouling can be reduced by decomposition of humic acids in the concentrate stream by O-3 oxidation. RESULTS: At a specific O-3 dose of 2.11 g O-3 per g COD (17.0 g m(-3) O-3 (g) for 20 min), a COD reduction of 38% and a hydrophobic COD reduction of 69% is achieved. The membrane permeability of the ozonated solution by NF 270 membranes is higher (20.6 x 10(-9) L s(-1) m(-2) Pa-1) than the permeability if the untreated solution is filtered (2.4 x 10(-9) L s(-1) m(-2) Pa-1). The COD retention of the ozonated solution was similar to the retention of the untreated samples. The addition of H2O2 allows a better mineralization degree, i.e. UVA removal increased from 53% to 66% if H2O2 was added as from 10 min oxidation at the same molar flow rate as O-3. CONCLUSION: O-3 oxidation can substantially alleviate membrane fouling by humic acids in nanofiltration systems and the addition of H2O2 can slightly improve its decomposition. (C) 2010 Society of Chemical Industry