Natural organic matter (NOM) has been considered a major contributor to the fouling of microfiltration (MF) and ultrafiltration (UF) membranes employed in water treatment. However, the fouling potential of NOM has often been assessed in terms of its size or chemical composition. The colloid's chemical properties have often been ignored. In this study, a chemical attachment-based (CAB) model established previously was used in conjunction with a variety of analytical techniques to investigate the existence of three major components of an aquatic NOM and their role in the fouling of a polyvinylidene fluoride MF membrane. The results suggest that colloidal NOM relevant to membrane fouling has a broader size distribution and variations in chemical properties than proposed previously. For the model aquatic NOM used in this research, fouling was primarily contributed by both non-humic and humic colloidal fractions. The non-humic colloids were larger in size and probably adhered to the membrane regardless of the solution chemistry, while humic colloids had variable size and stickiness depending on solution chemistry. The fouling caused by organic colloids was mostly hydraulically irreversible, as a consequence of favorable surface interactions. The CAB model provided a useful way to understand the role of organic colloids in membrane fouling. (C) 2008 Elsevier B.V. All rights reserved.