It has been shown that anionic membranes foul owing to the deposition of insoluble acidic colloids on the membrane surface facing the diluate. The fouling is caused by H+ ions generated by even minimal polarisation at the membrane surface. It proceeds autocatalytically because the precipitate forms a composite sandwich membrane together with its anionic base membrane and this sandwich membrane works in its closed direction, generating more H+. Fouling increases under polarising conditions, i.e. with decreasing salt concentration and decreasing flow velocity, with increasing current density and colloid concentration. It is much stronger on rough surfaced than on glossy surfaced membranes and stronger on membranes which show high dynamic polarisability. An alkaline pH decreases the humate type of fouling, especially in buffered solutions. Fouled stacks can be regenerated successfully by rinsing them with alkali, especially under reversed electric current.