Natural organic matter (NOM) plays a significant role in fouling of ultrafiltration membranes in drinking water treatment processes. The aim of this study was to obtain a better understanding of the interactions between the fractional components of NOM and a hydrophilic PES/PVP hollow fiber ultrafiltration membrane (150-200 kDa). NOM was fractionated into hydrophobic, transphilic and hydrophilic acid fractions according to the XAD-8/4 resin method proposed by Aiken et al. [1]. UF filtration tests were performed with diluted NOM fractions (1.5 mg/l DOC) isolated from surface water (Lake Ijssel). From NOM fractionation results, Lake IJssel water comprised about 53-55% hydrophobic, 20-22% hydrophilic and 25% transphilic NOM acids. Filtration of three NOM fractions (hydrophobic, hydrophilic and transphilic) suggested that the fouling potential was in the order: hydrophilic > hydrophobic > transphilic. The reversibility of fouling due to the hydrophilic fraction was very poor, and decreased from ca. 50% in the first cycle to < 30% after 6 cycles. LC-OCD (Liquid Chromatography-Organic Carbon Detection) analyses of UF feed and permeate showed almost no rejection of humic acids (20,000-1,000 Da), IOW molecular weight acids (< 350 Da) or neutrals/amphiphilic compounds (< 350 Da). The absence of a polysaccharide peak (Mw > 20,000 Da) in the LC-OCD chromatogram of the UF permeate, suggested that polysaccharides (> 20,000 Da) were rejected by the UF membrane and may have contributed to fouling.