Due to their hydrophobic pore systems and high surface areas, periodic mesoporous organosilicas (PMOs) offer unique properties for use as adsorbents of organic compounds. Two organosilicas (ethane-PMO and benzene-PMO) and a silica (PMS) were synthesized following a similar procedure. The adsorption of the herbicide S-Metolachlor from an aqueous solution was then compared on the three solids. Both PMOs exhibited a higher adsorption capacity than their silica counterpart. Additionally, both PMOs showed much higher stability than the PMS, whose mesostructure was seriously damaged under aqueous conditions. Moreover, both PMOs exhibited a very different adsorption behavior. The adsorption of S-Metolachlor on each material was described by different isotherms. The amount retained depended on the pH, and was maximum at pH = 2 for ethane-PMO and at pH = 4 for benzene-PMO. Our results suggest that both hydrophobic and aromatic pi-pi interactions play an important role in the adsorption of this herbicide. The adsorption capacity of benzene-PMO was very high with a loading of ca. 0.5 g herbicide per gram of adsorbent after successive adsorption cycles. (C) 2013 Elsevier B.V. All rights reserved.