This study aimed to evaluate the application potential of nanofiltration (NF) membranes for the rejection of organic acids in wastewaters. The rejection tests at different feed water chemistries, including pH and concentration, were performed on a laboratory scale using five different organic acids. The rejection of selected organic acids by the ES10 and NF270 membranes increased with increasing pH, depending on the degree of dissociation. The rejection of succinic and citric acids, which have molecular weights (M(W)s) larger than or closer to the molecular weight cutoffs (MWCOs) of employed NF membranes, was over 90% irrespective of operating pressure. Contrarily, the rejection of organic acids with M(W)s much smaller than MWCOs of the NF membranes increased gradually with increasing the applied pressure. The degree of organic acid rejection, except for formic acid, by the NF membranes was not significantly changed when altered in feed concentrations of 50-500 mg L-1. The rejection degree of charged organic solutes with M-W much smaller than MWCO of a NF membrane can be negatively affected by the presence of salts. Results of the short-term filtration test using an actual anaerobic effluent revealed that the NF270 efficiently removed organic acids from the wastewater. The increase of DOC rejection with filtration time could be explained by increase of electrostatic repulsion between membrane and dissociated organic acid and by membrane fouling. NF process showed considerable potential as an advanced wastewater treatment process for removing organic acids in wastewaters. (C) 2007 Elsevier B.V. All rights reserved.