The methanolysis of the insecticide paraoxon (2) was investigated in methanol solution containing varying [La(OTf)(3)] (OTf = -OS(O)(2)CF3) as a function of (s)(s)pH at 25 degreesC. Plots of the pseudo-first-order rate constants (k(obs)) for methanolysis as a function of [La(OTf)(3)](total) were obtained under buffered conditions from (s)(s)pH 5.15 to 10.97, and the slopes of the linear parts of these were used to determine the second-order rate constants (k(2)(obs)) for the La3+-catalyzed methanolysis of 2. Detailed analysis of the potentiometric titration data of La(OTf)(3) in methanol through fits to a multicomponent equilibrium mixture of dimers of general stoichiometry La-2(3+)(-OCH3)(n), where n assumes values of 1-5, gives the equilibrium distribution of each as a function of (s)(s)pH. These data, when fit to a second expression describing k(2)(obs) in terms of a linear combination of individual rate constants k(2)(2:1), k(2)(2:2)..... k(2)(2:n) for the dimers, allow one to describe the overall catalytic profile in terms of the individual contributions. The most catalytically important species are the three dimers La-2(3+)(-OCH3)(1), La-2(3+)(-OCH3)(2), and La-2(3+)(-OCH3)(3). The catalysis of the methanolysis of 2 is spectacular: a 2 x 10(-3) M solution of [La3+](total), at neutral (s)(s)pH, affords a 10(9)-fold acceleration relative to the base reaction (t(1/2) approximate to 20 s at (s)(s)pH 8.2) with excellent turnover. A mechanism of the catalyzed reaction involving the La-2(3+)(-OCH3)(2) species is proposed.