The kinetics of the La3+-catalyzed methanolysis of a series of S-aryl methylphosphonothioates (4a e, phenyl substituents = 3,5-dichloro, 4-chloro, 4-fluoro, 4-H, 4-methoxy) were studied at 25 degrees C with (s)(s)pH control. The reaction involves saturation binding of the anionic substrates to dimeric La3+/methoxide catalysts formulated as La-2(3+)(-OCH3)(x) where x = 2-5 depending on the solution (s)(s)pH. Cleavage of the La3+-bound methylphosphonothioates is fast, ranging from 5 x 10(-3) s(-1) to 5.5 x 10(-5) s(-1) or substrates 4a-e at a:pH of 8.4 and 1.6 x 10(-1) s(-1) to 4 x 10(-3) s(-1) at a(s)(s)pH of 11.7. The rate accelerations for the methanolysis of substrates 4a e, relative to their background methoxide-promoted reactions, average 7 x 10(10) and 1.5 x 10(9), respectively, at (s)(s)pH's of 8.4 and 11.7. The catalytic system is predicted to cleave FA 2192 (S-2(N,N-di-iso-propylaminoethyl) methylphosphonothioate), a toxic byproduct of the hydrolysis of VX, with a t(1/2) between 4 and 8 min at a :pH of 8.4, and 27 min at a (s)(s)pH of 11.7.