A kinetic study on nucleophilic substitution reactions of 4-pyridyl benzoate (2a) and O-4-pyridyl thionobenzoate (2b) with a series of cyclic secondary amines in acetonitrile at 25.0 degrees C is reported. Plots of pseudo-first-order rate constant (k(obsd)) vs. [amine] are linear and pass through the origin for the reactions of 2a but curve upward for those of 2b. The upward curvature observed for the reactions of 2b is typical for reactions that proceed through a stepwise mechanism with a zwitterionic intermediate T-+/-, which decomposes to the products via uncatalyzed and catalyzed routes competitively. The reaction of 2a has been suggested to proceed through a stepwise mechanism with T-+/-, in which expulsion of the leaving group occurs in the rate-determining step on the basis of a linear BrOnsted-type plot with (nuc) = 0.77. The catalyzed reaction of 2b from T-+/- has been proposed to proceed through a concerted mechanism with a six-membered cyclic transition state rather than via a stepwise pathway with an anionic intermediate T-. Factors influencing reactivity and reaction mechanism are discussed in detail.