Electro ultrafiltration is an effective method for mitigating membrane fouling. Herein, anti fouling mechanisms for the electro-ultrafiltration of natural organic matter (NOM) were investigated to illuminate the significance of NOM molecular polarity and weight. Results indicated that the membrane flux of NOM electro-ultrafiltration was greatly enhanced by electric field (EF) application, and that the increase in membrane flux magnitude for electro-ultrafiltration was more pronounced for NOM with higher hydrophilicity and larger molecular weight. The electro-ultrafiltration flux of the hydrophilic substances in NOM was enhanced by up to 20% after applying the voltage. The enhancement of membrane flux was more prominent for the electro-ultrafiltration of humic acid (HA) solution with high ionic strength and coexisting Ca2+. With increasing EF strength in the electro-ultrafiltration of HA, membrane flux increased at first due to the electrokinetic migration of negatively charged HA, and then declined for fields larger than the critical EF, where electro-ultrafiltration reached maximum flux. The HA molecules became polarized under a high EF, which induced aggregation between the molecules and increased affinity between HA and the membrane. This facilitated the formation of a dense cake layer, leading to relatively severe membrane fouling under EF strengths larger than the critical EF. (C) 2018 Elsevier Inc. All rights reserved.