Natural organic matter (NOM) characteristics were determined for three ground waters exhibiting different water quality conditions. The water quality of the three feed waters collected at various water table depths was characterized by XAD-8/-4 resin adsorption, high performance size exclusion chromatography with ultraviolet and dissolved organic carbon (DOC) detections, and Fourier transform infrared spectroscopy (FTIR) to determine NOM fractionation, molecular weight, and NOM functional groups, respectively. Systematic studies were conducted to identify potential NOM foulants in ground water for nanofiltration (NF) membrane fouling. The results show that the hydrophobic fraction of NOM in all of the samples was significantly high (71-93%) compared to the hydrophilic (1.7-22.6%) and transphilic (5.3-6.6%) fractions. However, insignificant flux-decline (less than 5%) was observed for the highest DOC (36.9 mg l(-1)) and hydrophobic NOM (93%) containing groundwater compared to the other lesser DOC and hydrophobic NOM containing ground waters. This is presumably due to either higher fractions of hydrophilic and transphilic NOM or inorganic interactions that may be major foulants. Based on FTIR, aromatic foulants were observed at 1662 cm(-1) (CO-NH2 or CO conjugated with aromatic rings) for the fouled NF membrane with the relatively low DOC source waters. The contact angle of the clean membrane (52 degrees ) decreased with fouling up to 42-47 degrees for fouled membranes with the various samples.