The capacities of two Moroccan clays, one of them swelling and the other non-swelling, as sorbents of three ionisable pesticides: terbutryn (basic), dicamba (anionic) and paraquat (cationic) were studied. Sorption was studied on natural and modified clays with three quaternary alkyl ammonium cations of different long-chain. octadecyltrimethylammonium (ODTMA), octyltrimethylammonium (OTMA) and tetramethylammonium (TMA). Freundlich or Langmuir models were used to obtain sorption parameters. Sorption of terbutryn and dicamba by ODTMA-clays was always higher than by natural clays. Distribution coefficients (Kd) of these modified clays increased 55-128-fold (terbutryn), and 1.4-8.6-fold (dicamba) respect to the natural clays. In contrast, sorption of terbutryn by the TMA-clays and of dicamba by TMA- and OTMA-clays was lower than for natural clays. Sorption coefficients of paraquat by natural clays were similar or higher than those obtained with modified clays. In general, the sorption of herbicides was higher by swelling clay than by non-swelling clay. The results point out that the Moroccan clays studied may be used as sorbents of cationic pesticides in their natural form and of basic or even anionic herbicides when modified with long-chain organic cations in barrier technologies to enhance the immobilization of herbicides and to prevent the contamination of surface and ground water. (C) 2009 Elsevier B.V. All rights reserved.