The adsorption of the herbicide Alachlor on montmorillonite saturated with different cations (Al3+, Cu2+, Ca2+, Mg2+NH4+, Na+, Li+, Rb+, and Cs+) was studied in aqueous suspension. Equilibrium adsorption isotherms at 5° and 22°C fit the Freundlich relationship. A correlation between adsorption and the polarizing power of the exchangeable cation was observed. A decrease in temperature from 22° to 5°C resulted in an increase in adsorption except for Al-, Rb-, and Cs-montmorillonite. The isosteric heats of adsorption were also determined. The interaction of Alachlor with homoionic montmorillonites was also studied by FT-IR spectroscopy in an organic solvent. The results indicated that the molecule is adsorbed on monoionic montmorillonite by a coordination bond, through a water bridge, between C==O groups and the exchangeable cation of the clay. Further, the coordination strength is directly correlated with the polarizing power of the cation. X-ray diffraction analyses showed that the herbicide was able to penetrate the interlayer space of montmorillonite saturated with polyvalent cations. On moderate heating (70°C for 6 h) Alachlor adsorbed on Al-, Cu-, Ca-, Mg-, and NH4-clay is partially decomposed to 2-chloro-2′,6′-diethylacetanilide.