Contact angles, zeta potentials, and IR spectra of clean and NOM-fouled membranes were compared to demonstrate differences in hydrophobicity, surface charge, and functional groups, respectively. Contact angle increased and decreased for membranes fouled with hydrophobic and hydrophilic NOM, respectively. Zeta potential decreased negatively (less negative) for membranes fouled with both hydrophobic and hydrophilic NOM, suggesting that ionizable functional groups were coated with non-ionizable NOM components (possibly neutrals). The IR spectra reduced in absorbance intensity for NOM-fouled membranes, suggesting that aromatic groups were coated with non-aromatic or less aromatic NOM components. The results of XAD-8/4 fractionations of: foulants (desorbed NOM with NaOH) showed that hydrophilic and hydrophobic NOM components were the major foulants for membranes fouled with hydrophilic and hydrophobic NOM-source waters, respectively.