The Eu3+-promoted methanolysis of three esters, p-nitrophenyl acetate (1), phenyl acetate (2), and ethyl acetate (3) is reported, as well as the potentiometric titration of Eu3+ in MeOH at various [Eu(SO3CF3)(3)] (SO3CF3 = OTf). The titration data are analyzed in terms of two ionizations corresponding to macroscopic (s)(s)pK(a)(1) and (sPKa2)-P-s values, which are respectively defined as the 'pH values at which the [CH30 CH3O-]/[Eu3+] = 0.5 and 1.5. As a function of increasing [Eu(OTf)(3)], (s)(s)pK(a)(1) increases slightly due to a proposed Eu3+/-OTf ion pairing effect, which tends to reduce the acidity of the metal-coordinated CH3OH, while (s)(s)pK(a)(2) decreases due to the formation of Eu3+ dimers and oligomers which stabilize the (Eu3+(CH3O-)(2))(n) forms through bridging of the methoxides between two or more metal ions. For ester 1, a detailed kinetic analysis of the reaction rates as a function of both [Eu(OTf)(3)] and (s)(s)pH in buffered methanol reveals that the (s)(s)pH/second-order rate constant (k(2)) plot for the catalyzed reaction follows a bell-shaped profile, suggesting that the active form is a Eu3+(CH3O-) monomer with a kinetic (s)spK(a)(1) of 6.33 +/- 0.06 for formation and a (s)(s)pK(a)(2) of 8.02 +/- 0.10 for its conversion into the inactive (Eu3+(CH3O-)(2))(n) oligomeric form. At higher (s)(s)pH values, plots of k(obs) vs [Eu(OTf)(3)] are linear at low metal concentration and plateau at higher metal S concentration due to the formation of inactive higher order aggregates. The Eu3+(CH3O-) catalysis of the methanolysis of esters 1, 2, and 3 is substantial. Solutions of 10(-2) M of the catalyst at (s)(s)pH 7.12 accelerate the reaction relative S to the methoxide reaction at that (sPH)-P-s by 8 530 000-, 195 000 000- and 7 813 000-fold, respectively.