Infrared spectroscopy was used to study the hydrolysis of atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine] upon interaction with homoionic soil colloids. Montmorillonite, an allophanic soil clay, and a montmorillonitic Coker soil clay were saturated with H+, Al3+, Cu2+, and Ca2+ and treated with atrazine and hydroxyatrazine [2-hydroxy-4-(ethylamino)-6-(isopropylamino)-s-triazine]. Hydrolysis of atrazine was evaluated by the presence of a strong hydroxyatrazine carbonyl absorption band at 1745 cm-1. The H+- and Al3+-saturated montmorillonite and montmorillonitic Coker soil clay promoted atrazine hydrolysis while Ca2+- or Cu2+-saturated montmorillonite did not. A small degree of atrazine hydrolysis was detected in the Cu2+-Coker soil clay. Dehydration of Ca2+- or Cu2+-Coker soil clay after equilibration with atrazine increased the hydrolysis of atrazine. The allophanic soil clay did not catalyze the hydrolysis of atrazine when the exchange complex was saturated with H+, Al3+, Ca2+, or Cu2+. Moreover, Al3+-allophane was not sufficiently acidic to protonate hydroxyatrazine. Thus, a major difference exists between soil allophanic colloids, montmorillonitic soil clays, and montmorillonite as catalysts in the protonation and hydrolysis of atrazine.