Pretreatment of seawater by adsorption onto low-cost and readily available materials is very important for guaranteeing the viability of desalination by reverse osmosis. For this purpose, the adsorption capacity of a modified Algerian montmorillonite was studied. A comparison with a similarly modified commercial Montmorillonite K-10 (Fluka) was also made. Modification of the montmorillonitic matrices was achieved by inorgano-organic pillaring. Adsorption performances were tested for synthesized seawater (TDS =37 g/l) doped with humic acid at 5 mg/l. Kinetic study results revealed better stability of the Algerian montmorillonite with respect to organic matter and salts. On the other hand, pH variation greatly influenced the interactions between the adsorbent material and the HA macromolecule. At pH 3, the kinetic study revealed a 60% elimination of humic matter as early as the first 30 min. Afterwards, it reached a maximum value of 70% for an equilibrium time of 3 h. An unexpected effect was the simultaneous substantial conductivity decrease reaching 35% in the presence of the combined pillared montmorillonite (Mont-Al-CTAB) at pH 3. It seems that the presence of a cationic tensioactive constituted of amphiphilic molecules is a source of interaction between cations and organic molecules. Moreover, at pH 3, which is below 3.5 corresponding to the value of the point zero charge of Mont-Al-CTAB, cationic exchanges did exist.