Colloidal fouling causes serious problems in many membrane plants. Two different kinds of flux-reducing phenomena occur when treating colloidal dispersions, When treating stable dispersions the flux is reversible and can be restored after changing the operating parameters, such as the transmembrane pressure or the crossflow velocity. The flux reduction experienced when treating unstable colloidal dispersions is irreversible. In this study the DLVO theory, well-known from colloid chemistry, has been used to illustrate the mechanisms underlying the difference in filtration characteristics between stable and unstable colloidal dispersions.