The reactions of acetyl halides, CH3C(=O)X and corresponding sulfur analogues, thioacetyl halides, CH3C(=S)X, where X = F and Cl, with NH3 nucleophile were studied theoretically, at the QCISD level of theory, in the gas phase and in aqueous solution. All reactions occurred via the tetrahedral species, and reactions through neutral intermediates both in the gas phase and in aqueous solution could be ruled out, except for the case of the gas-phase reaction of acetyl fluoride. The tetrahedral structure was a transition state (TS) in the reactions of acetyl chloride, while it was a stable intermediate in reactions of thioacetyl halides. These differences could be caused by the pi-bond strength of C=O and C=S. In the case of acetyl fluoride, the T-+/--type species was neither a saddle point nor an energy minimum in the gas phase, but existed as a stable intermediate in aqueous solution due to solvation. Moreover, in reactions of thioacetyl chloride, the rate-limiting step changed from the first step in the gas phase to the second step in aqueous solution, since the zwitterionic intermediates become more stabilized in aqueous solution. However, lower activation energies (Delta G(+/-)) in aqueous solution were not caused by the solvent effects, but smaller deformation effects, in going from reactants through the TS.