Second-order rate constants (k N) have been measured spectrophotometrically for reactions of 2,4-dinitrophenyl X-substituted benzoates (X = 4-MeO, H and 4-NO 2) with a series of Z-substituted pyridines in 80 mol % H 2O/20 mol % DMSO at 25.0 ± 0.1 °C. The Brønsted-type plots exhibit downward curvature (e.g., β 2 = 0.89 ∼ 0.96 when pK a < 9.5 while β 1 = 0.38 ∼ 0.46 when pK a > 9.5), indicating that the reaction proceeds through a stepwise mechanism with a change in rate-determining step (RDS). The pK a°, defined as the pK a at the center of Brønsted curvature, has been analyzed to be 9.5 regardless of the electronic nature of the substituent X in the benzoyl moiety. Dissection of k N into the microscopic rate constants k 1 and k 2/k -1ratio has revealed that k 1 is governed by the electronic nature of the substituent X but the k 2/k -1 ratio is not. Theoretical calculations also support the argument that the electronic nature of the substituent X in the benzoyl moiety does not influence the k 2/k -1 ratio.