Fulvic acid (FA) is the main water-soluble component in humic substances which usually cause membrane fouling in the drinking water treatment process. This study investigates the ultrafiltration membrane fouling characteristics and mechanisms caused by FAs in the presence of various salt ions (Ca2+, Mg2+, K+ and Na+). A series of experiments had been carried out to characterize the decline of permeate flux, pollutant rejection, FA aggregates, and the morphology of the membrane's fouling layer. Compared with pure FA solution, FA-monovalent solution containing cations (K+ and Na+) caused much lower permeate flux and smaller amount of fouling layer, which was difficult to remove by flushing with water. The presence of divalent cations (Ca2+ and Mg2+), caused the permeate flux to noticeably increase and membrane fouling can be easily removed. Laser particle size analysis showed that the FA formed larger aggregates in the presence of Ca2+ or Mg2+ cations. Based on an atomic force microscope and scanning electron microscope analysis, results showed that monovalent cations made the colloids form a gel layer, which led to pore blocking and permeate flux decrease. And divalent cations reacted with FA and formed a cake layer of larger particles creating porous structures on the membrane surface. (C) 2014 Elsevier B.V. All rights reserved.