This study examined the effect of molecular weight (MW) fractions on electro-ultrafiltration (EUF). MW fractions fractionated from Aldrich humic substances (AHSs) were defined as G1 (3000 to >20,000 Da), G2 (500 to 3000 Da) and G3 (<400 to 500 Da) by using gel fractionation chromatography (GFC). G3 exhibited the largest flux decline in the absence of an electric field. The decline may be attributed to the relatively small MW and apparent electrophoretic mobility (AEM) of G3. The result from atomic force microscopy (AFM) roughness analysis suggested that G3 possibly blocked the membrane pore, but G1 and G2 were possibly accumulated on the membrane surface. In the presence of an electric field, both flux and removal were simultaneously increased due to the electrokinetic phenomena. However, the extent of the flux decline was higher for G1 than that for G3. It was postulated that electrochemical reactions changed the characteristics of membrane and AHSs. (C) 2008 Elsevier B.V. All rights reserved.