Kinetic studies of the reaction of O-methyl-S-phenylthiocarbonates with benzylamines in methanol at 45.0 ℃ have been carried out. The reaction proceeds by a stepwise mechanism in which the rate-determining step is the breakdown of the zwitterionic tetrahedral intermediate, $T^{pm}$, with a hydrogen-bonded four-center type transition state (TS). These mechanistic conclusions are drawn based on (ⅰ) the large magnitude of ${ ho}_X;and;{ ho}_Z$, (ⅱ) the normal kinetic isotope effects $(k_H/k_D;>;1.0)$ involving deuterated benzylamine nucleophiles, (ⅲ) the positive sign of ${ ho}_{XZ}$ and the larger magnitude of ${ ho}_{XZ}$ than that for normal $S_N2$ processes, and lastly (ⅳ) adherence to the reactivity-selectivity principle (RSP) in all cases.