The kinetics and mechanism of the reactions of O-ethyl S-(Z)aryl thiocarbonates with (X)benzylamines in acetonitrile at 45.0ЊC are studied. Relatively small values of ␤ X (␤ nuc ) ϭ 0.6 ϳ 0.8 and ␤ Z (␤ lg ) ϭ Ϫ0.5 ϳ Ϫ0.7 together with a negative cross-interaction constant XZ (ϭ Ϫ0.47) and failure of the reactivity-selectivity principle (RSP) are interpreted to indicate a concerted mechanism. The normal kinetic isotope effects (k H /k D ϭ 1.3 ϳ 1.8) involving deuterated benzylamine nucleophiles suggest a hydrogen-bonded, four-center-type transition state.