Ground water pollution due to herbicide leaching has become a serious environmental problem. Imazaquin [2-(4-isopropyl-4-methyl-5-oxo-2-imidazolin-2-yl)quinoline-3-carboxylic acid] is an herbicide used to control broadleaf weeds in legume crops. Imazaquin is negatively charged at the basic pH of calcareous soils and exhibits high leaching potential in soils. Our aim was to design formulations of imazaquin to reduce herbicide leaching. Imazaquin sorption on pillared clay (PC) and crystal violet (CV)-montmorillonite complexes was studied. The CV-montmorillonite complexes become positively charged with adsorption of CV above the cation exchange capacity (CEC) of montmorillonite, and thus can sorb imazaquin. The Langmuir equation provides a good fit to isotherms of imazaquin sorption on PC and CV-montmorillonite complexes, but for charged complexes an equation that combines electrostatics with specific binding was preferred. Maximal imazaquin sorption was 17.3 mmol kg-1 for PC and 22.2 mmol kg-1 for CV-montmorillonite complexes. The extents of imazaquin desorption into water were 21% for PC and 5% for CV-clay complexes. The presence of anions decreased imazaquin sorption on both sorbents in the sequence phosphate > acetate > sulfate. Reduction of imazaquin sorption by the anions and the extent of its desorption in electrolyte solutions were higher for PC than for CV-clay complexes. Leaching of imazaquin from CV-montmorillonite formulations through soil (Rhodoxeralf) columns was two times less than from PC formulations and four times less than that of technical imazaquin. The CV-montmorillonite complexes at a loading above the CEC appear to be suitable for preparation of organo-clay-imazaquin formulations that may reduce herbicide leaching significantly.