The SN2 displacements of chloride ion from CH3Cl, C2H5Cl, and C2H4Cl2 by acetate and hydroxide ions have been investigated, using ab initio molecular orbital theory at the HF/6-31+G(d), MP2/6-31+G(d), and MP4/6-31+G(d) levels of theory. The central barriers (calculated from the initial ion-molecule complex) of the reactions, the differences of the overall reaction energies, and the geometries of the transition states are compared. Essential stereochemical changes before and after the displacement reactions are described for selected cases. The gas phase reactions of hydroxide with CH3Cl, C2H5Cl, and C2H4Cl2 have no overall barrier, but there is a small overall barrier for the reactions of acetate with CH3Cl, C2H5Cl, and C2H4Cl2. A self-consistent reaction field solvation model was used to examine the SN2 reactions between methyl chloride and hydroxide ion and between 1,2-dichloroethane and acetate in solution. As expected, the reactions in polar solvent have a large barrier. However, the transition state structures determined by ab initio calculations change only slightly in the presence of a highly polar solvent as compared with the gas phase. We also calibrated the PM3 method for future study of an enzymatic SN2 displacement of halogen.