A conceptual model for the fouling of MF membranes by aquasols (fine particles in aquatic environments) has been developed and tested. The focus of the work is on the impacts of particle size and colloidal stabilities (both particle-particle and particle-membrane). The model predicts that fouling is most severe when aquasol particles are unstable with respect to attachment to the membrane. Meanwhile, their stability with respect to mutual aggregation has competitive effects with attachment in deposition and usually causes a decrease of total fouling. The size of aquasol particles plays a secondary role although it can determine the extent of fouling caused by unstable aquasols. There is also a critical range of particle sizes relative to membrane pore diameter that can lead to efficient pore blocking. The model findings were validated experimentally using polystyrene latex spheres and bovine serum albumin protein, and a polyvinylindene fluoride membrane. These results are consistent with the findings of other experimental studies. The model developed in this study provides a mechanistic explanation for the widely observed role of aquasols in the MF of natural waters and wastewaters. (C) 2008 Elsevier B.V. All rights reserved.