The study was carried out with the objective of developing suitable and sustainable low cost adsorbent materials for diazinon, an organophosphate pollutant used as a pesticide. Montmorillonite modified with iron was used. Two different types of iron-montmorillonite, each having different contents of iron and synthesized with different pH and levels of Fe hydrolysis were used. One was denoted “Fe-modified” and the other denoted as “FeOH-modified”. The color of the samples changed from greyish green to light-reddish brown after the modification. X-ray diffraction and physical observations were used for characterization of the samples. The d-spacing of the samples was greater than 15 ?, indicating the formation of iron hydroxides in the interlayer space of montmorillonite. The amount of adsorption was calculated from the difference between the initial and the final concentration of diazinon. The adsorption data were analyzed using the Langmuir adsorption isotherms. The amounts of diazinon adsorbed were 58.8 and 54.1 mmolkg^-1 for Fe-modified and FeOH-modified respectively. The steep rise in their adsorption isotherms indicated the possibility of adsorption for low level of diazinon in polluted water.