The gas-phase identity methyl transfer reactions, X- + CH3X reversible arrow XCH3 + X-, have been investigated with X = H, F, Cl and Br at the MP2, B3LYP, QCISD and QCISD(T) levels by geometry and energy optimizations using the 6-311++G(3df,2p) basis sets at each level. Energy barriers, DeltaE(elec)(double dagger), DeltaEZPE(double dagger), DeltaH(double dagger) and DeltaG(double dagger), are reported relative to both the reactants (DeltaG(double dagger)) and ion-dipole complex levels (DeltaG(c)(double dagger)). The electron correlation 0 Z energy (-E-corr) decreases in the MP2, QCISD and QCISD(T) results as the size (number of electron) of the system becomes larger (X = F --> Cl --> Br). The MP2 and QCISD methods underestimate the electron correlation effects relative to the highest level QCISD(T) results, which are, in general, in good agreement with the available experimental values. The lowest and highest activation barriers obtained with X = F and H, respectively, are found to be the consequences of the strong electrostatic interaction energies in the TS (DeltaE(es) much less than 0 and DeltaE(es) much greater than 0, respectively), in contrast to small differences between nucleophiles, X, in the proximate sigma-sigma* charge transfer and deformation energies. The gas-phase barrier heights are in the order X F < Br < Cl < H, and hence the reactivity and the gas-phase nucleophile strength are in the reverse order. Moreover, the extent of bond formation in the transition state, as expressed by the percentage bond order change, %Deltan(double dagger), is also in the order of intrinsic nucleophilicity. Thus the stronger the nucleophile, the greater is the bond formation in the transition state for the intrinsic barrier controlled reactions.