Three ultrafiltration membranes [thin-film composite, polyethersulfone (PES), and sulfonated PES] which have different chemistries and nominal relative molecular mass cut-offs (MWCO), were compared in terms of their characteristics of flux; decline, rejection of natural organic matter (NOM), and the adsorbed foulants, with two very different (relatively hydrophilic and hydrophobic) NOM-contaifiing source waters. To facilitate comparison, the ratio (J(o)/k) of the initial pure water flux J(o) to the estimated boundary layer mass transfer coefficient k was used to define similar initial hydrodynamic operating conditions for the three different membranes, The membranes showed differences in NOM rejection when filtering hydrophilic NOM-source water, and also exhibited differences in flux decline with the hydrophobic NOM-source water. Flux decline and NOM-rejection were quantified using a resistances-in-series model-and effective molecular mass cut-off, respectively. Non-charged NOM fractions (hydrophilic and hydrophobic neutrals/bases) were found to be significant foulants for these negatively charged membranes.