This paper looks into the problem of fouling of UF/MF membranes due to NOM present in surface waters. NOM can be divided into particulate, colloidal and dissolved fractions. The idea here is to fractionate the surface water into the above-mentioned fractions, and to test, the fouling characteristics of each fraction on a MF membrane, with and without pre-treatment by coagulation. Coagulation was performed at a pH of 7.5, using poly aluminium chloride (PAC) (2.5 mg/LAl3+), and the flocculated solution was slowly transferred into the feed tank of the filtration unit. A lab-scale single membrane capillary (SMC) test unit was used for the filtration test runs. The bulk surface water was fractionated using 0.45 mu M (MF), 5 kD (UF) and/or 3.5 kD dialysis bags. Membrane performance was measured using permeability decline and its recovery after each backwash. Feed and permeate samples were analysed using UVA(254) (m(-1)) to estimate the organics removal through filtration. It could be shown that (1) particulates (organic/inorganic) helped minimise irreversible fouling, (2) the colloidal organics are the most irreversible fouling causing fraction, and (3) coagulation pre-treatment plays a significant role in improving and stabilising membrane performance.