The problem of membrane fouling by natural organic matter (NOM) is commonly experienced in membrane filtration of natural waters for drinking water treatment. This study continues exploration of a novel pretreatment to reduce membrane fouling using a colloidal polysulfone (PSf) adsorbent that removes compounds that would normally adsorb on membranes [M.M. Clark, W.Y. Alm, X. Li, N. Sternisha, R.L. Riley, Formation of polysulfone colloids for adsorption of natural organic foulants, Langmuir 21 (2005) 727-7213.]. Pretreatment of high-fouling lake water samples yields continued decrease in adsorptive fouling of 20-kDa molecular weight cut-off ultrafilters as the colloid concentration is increased incrementally from 5 to 100 mg/L; it is also shown that kinetics of adsorption of the critical NOM foulant by PSf colloids is quite fast in the water treatment context, occurring within minutes of contact with the water samples. Supporting previous work, measurements of dissolved organic carbon and apparent molecular weight distribution reveal that only a small fraction of the NOM is removed by PSf colloids, hence, only a small fraction of NOM causes fouling. High-pressure size-exclusion chromatography suggests that the foulant has an apparent molecular weight ranging from 20 to 200kDa. In addition, study of the foulants adsorbed on the colloids using Fourier-transform infrared spectrometry and energy-dispersive spectroscopy indicates the presence of trace amounts of silicon, indicating a role of silicon in NOM adsorption. (c) 2006 Elsevier B.V. All rights reserved.