Three archetypal ion pair nucleophilic substitution reactions at the methylsulfenyl sulfur atom LiX + CH3SX -> XSCH3 + LiX (X = Cl, Br, and I) are investigated by the modified Gaussian-2 theory. Including lithium cation in the anionic models makes the ion pair reactions proceed along an S(N)2 mechanism, contrary to the addition-elimination pathway occurring in the corresponding anionic nucleophilic substitution reactions X- + CH3SX -> XSCH3 + X-. Two reaction pathways for the ion pair S(N)2 reactions at sulfur, inversion and retention, are proposed. Results indicate the inversion pathway is favorable for all the halogens. Comparison of the transition structures and energetics for the ion pair S(N)2 at sulfur with the potential competition ion pair S(N)2 reactions at carbon LiX + CH3SX -> XCH3 + LiXS shows that the S(N)2 reactions at carbon are not favorable from the viewpoints of kinetics and thermodynamics.