A major factor limiting the use of nanofiltration (NF) in the water treatment process is membrane fouling by natural organic matter. Two NF membranes, Desal 5 DK and DL, which have different hydraulic permeabilities, were compared. Solution chemistries selected for this investigation were concentrated on the common salts found in a variety of wastewaters, including monovalent ions (Na+, Cl-), divalent hardness cations (Ca2+, Mg2+) and divalent anions (SO42-). Results of fouling experiments with humic acids demonstrated that the condition of hydrodynamics and the chemical compositions of the feed solutions could significantly influence the fouling tendency. Higher crossflow velocity and lower initial flux mitigated accumulation of humic acid (HA) on the membrane to some extent. The adsorption of humic substances onto the membrane surface was markedly enhanced in the presence of divalent cations due to the formation of HA-complexed divalent cations. The experiment also indicated that the membrane surfaces preferred to adsorb divalent cations rather than divalent anions because the membrane had a negative charge in the higher pH range investigated. In addition, choosing a proper divalent cation chelating agent as a cleaning reagent, especially at basic conditions, can improve the membrane process by dissolving and removing the HA fouling substance.