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Methyl parathion (O,O-dimethyl O-(4-nitrophenyl) phosphorothioate) adsorption was studied on montmorillonite, kaolinite, halloysite, natural zeolite, ion exchange resins and calcium carbonate. Methyl parathion was highly adsorbed by montmorillonite, followed by zeolite, and very little adsorption was obtained on kaolinite and halloysite. Calcium carbonate did not exhibit any adsorption. The values of the partial molar free energy "delta G" were calculated for all systems. For the montmorillonite-methyl parathion system, the calculated partial molar heat of adsorption "delta H", and the conformity of the data to Freundlich equation indicated a possible physical mechanism of adsorption. Increasing acetone concentration decreases methyl parathion adsorption and dehydration increased adsorption in the nonaqueous system. Moreover, the increased adsorption on the swollen clay indicated that methyl parathion was adsorbed on the interlamellar surfaces of the clay. This was also in agreement with the results of the desorption studies, since the insecticide was not desorbed using the same aqueous acetone solution. This indicated that methyl parathion was adsorbed as a water-insoluble organic compound.