Nanofiltration polyester thin-film composite membranes have been prepared by interfacial polymerization using commercial polyethersulfone membrane support. Different monomer bisphenol A (BPA) concentrations in the aqueous solution and various interfacial polymerization times in the organic solution containing trimesoyl chloride (TMC) were studied. The success of the conducted interfacial polymerization procedure was corroborated by FTIR-ATR. Irreversible fouling of both the unmodified polyethersulfone and the modified polyester thin-film composite polyethersulfone membranes have been studied using humic acid model solutions at different pH values. It was observed that polyester thin-film composite membranes exhibited practically no tendency to be irreversibly fouled by humic acid molecules at neutral environment. However, the permeate flux was decreased and the irreversible fouling factor was enhanced with decreasing the pH to a value of 3. (C) 2009 Elsevier B.V. All rights reserved.