In this study, ultrafiltration (UF) membranes with various pore sizes were prepared by the phase inversion method, and their fouling characteristics were investigated with membrane bioreactor (MBR) mixed liquor. MBR sludge was fractionated into three parts, suspended solids, colloids and solutes, and their individual contributions to membrane fouling were quantified in order to asses the mechanisms by which fouling occurs. Cake layer formation on the membrane surface constituted the main fouling mechanism and the incidence of irreversible fouling was so small as to be negligible. The degree of fouling correlated very strongly with membrane permeability. This implies that hydrodynamic conditions are important factors affecting membrane fouling. The fouling contribution of each sludge fraction appeared to depend on particle size, as both permeation drag and back transport velocity are particle size-related functions. Solutes played a significant role in the initiation of cake layer formation, because they were deposited onto the membrane surface and pore wall immediately upon initial filtration, but were dislodged only in small amounts by cross flow. Suspended solids were consistently deposited onto the membrane surface, until flux reached a steady state and colloids exhibited characteristics commensurate with an intermediated state between solutes and suspended solids. Suspended A solids were, in fact, found to be the main contributor to the fouling process. However, the relative contribution of each of the sludge fractions to membrane fouling varied with the permeability of the membrane, and also with the hydrodynamic condition. (c) 2005 Elsevier B.V. All rights reserved.