Increasingly stringent regulations for drinking water quality have stimulated the application of ultrafiltration to water treatment. In addition to removing particulate materials from water (including microorganisms, bacteria and viruses), the use of membrane treatment also meets purification requirements. However, irreversible fouling curtails the economic viability of such a process. Experiments in stirred-cells were conducted to evaluate the effects of surface water composition on rejection and fouling of two ultrafiltration membranes with different molecular weight cut-offs (10 and 100 kDa). Experimental solutions consisted of natural organic matter or humic substances in a background electrolyte. The effect of calcium concentration decreased rejection of humic acid under certain circumstances. This is believed due ro reduced molecular size with an initial increase in calcium concentration. However, at about 2.5 mM CaCl2, IHSS humic acid aggregates. This aggregation increased rejection, and also caused irreversible fouling of the 100 kDa membrane, presumably as a result of pore size reduction due to internal deposition of aggregates. This was confirmed by blocking law analysis. The variation of transmembrane pressure indicated the importance of a 'critical flux' effect. The organics and their various fractions showed differences both in rejection and flux decline. The larger and more UV-absorbing fraction of humic acid was shown to be responsible for irreversible pore adsorption and plugging. The fulvic acid and the hydrophilic fraction showed a smaller and mostly reversible flux decline.