The interaction of aminotriazole (AMT) at pH 4 on Wyoming montmorillonite (mainly with Na ions) and Mg-vermiculite has been studied by X-ray diffraction and infrared spectroscopy. The AMT is adsorbed on montmorillonite in the cationic form by cation exchange. The amount of pesticide adsorbed was 71 mEq/100 g, which comprises approximately 91% of the CEC of this sample (78.2 mEq/100 g). Saturation was reached in 24 h. giving rise to a complex with basal spacing 12.5 Aa. Vermiculite adsorbs 167 mEq/100 g, almost 20% greater than the CEC (141 mEq/100 g), and the basal spacing was stabilized at 13.68 Aa after five weeks of treatment with AMT. A part of the AMT is adsorbed in cationic form. displacing a great part of the exchangeable Mg (super 2+) cations. The rest is adsorbed in molecular form by coordination to the Mg (super 2+) cations which remain in the interlamellar space, removing a great amount of water, and remaining in the interlamellar space of vermiculite after washing with water, probably because of a steric hindrance from the AMT cations adsorbed.