Although natural organic matter (NOM) is known to adsorb on filtration membranes, little is known about whether membranes possess an adsorption capacity for NOM. In this research, 4000 mL of particle-free lake water containing dissolved NOM was filtered through a 0.2-mu m polypropylene (PP) microfiltration membrane. Permeate was collected over 10 batches of 400 mL each. A 100-mL sample from each batch of permeate was then filtered through a fresh 20,000 Da molecular weight cut-off (MWCO) polyethersulfone (PES) ultrafiltration membrane. It was observed that later batches of microfiltration permeate caused more ultrafiltration membrane fouling than earlier batches of permeate. This suggests an adsorption capacity for the PP microfilter, and this was determined to be about 14 mg of DOC adsorbed per gram of PP membrane. A mass balance across the microfilter revealed that about 85 % of DOC in the lake water did not cause fouling. Used PES ultrafilters were analyzed using scanning electron microscopy, and while there was a slight increase in material deposited on the ultrafilter for later batches of microfiltration permeate, a clean membrane surface was apparent in all micrographs. This suggests that the NOM responsible for membrane fouling was smaller than the pores of the 20,000 Da PES ultrafiltration membrane. These findings provide useful insights into membrane fouling, and suggest that adsorptive fouling should be considered in the design of membrane processes used to filter natural waters. (c) 2005 Published by Elsevier B.V.