Effects of carbon black (CB) addition on membrane fouling and rejection of macromolecular humic acids (HA) were evaluated by a stirred-cell ultrafiltration unit. Stable CB dispersions increased filtration resistances, but enhanced HA rejection by the membranes. Monovalent and divalent ions affected the filtration resistance of CB solution differently; namely, NaCl solution showed a very high resistance due to the concentration of CB in the diffusion boundary layer near the membrane surface, whereas CaCl2 and MgCl2 solutions showed only cake resistance. The cake layer containing both CB and HA was more easily removed from the membranes than HA-cake layer.