The mechanism for the hydrolysis of methyl parathion (MP) by a bifunctional quaternary-ammonium based long-chained organclay (LCOC) containing an alkylamine (-CH2-CH2-NH2) headgroup was elucidated. The pathway of the catalytic hydrolysis of methyl parathion by the LCOC was defined by following the effect of replacing H2O with D2O, by replacing the primary amino headgroup by a tertiary amino group, and by a detailed mathematical analysis of the proposed reaction scheme. A phosphorothioate isomer of MP was formed in the presence of the LCOC as an intermediate reaction product, initially increasing in concentration and then disappearing. The isotope effect was minimal and substituting a tertiary amine in the LCOC increased the rate of MP hydrolysis. A mechanism is proposed in which hydrolysis of MP can proceed via both a direct route (specific base hydrolysis) and through the formation of the isomer which then undergoes specific base hydrolysis more rapidly than the parent MP. The relative importance of each pathway is a function of pH with the direct hydrolysis of MP being predominant at higher pH values (pH > 10) and the isomer intermediate pathway predominating at lower pH values (pH ~8-10)