The gas-phase acyl transfer reactions X- + RCOY reversible arrow RCOX + Y- with R = H, CH3 and X, Y = Cl, Br have been investigated with MO theory at the G2(+)MP2 level. Attempts have been made to locate two types of adducts, a tetrahedral adduct formed by an out-of-plane pi-attack (pi-adduct) and an adduct formed by an in-plane sigma-attack (sigma-adduct). In all cases, the sigma-type adducts are nonexistent. Careful examination of the energies (Delta E) at the MP2/6-311+G**: level shows that the transition structure region is very flat with a very low barrier for decomposition of the intermediate. This small barrier becomes inverted (i.e,, the intermediate level is higher than the transition state) as the zero-point and thermal energy corrections are applied, which is eventually restored to the normal barrier (delta Delta G = Delta G(TS) -Delta G(Int) congruent to -0.9 kcal mol(-1)) when entropy effect is accounted for, The pi-adducts are stabilized mainly by the proximate second-order sigma-sigma* type charge transfer interactions. The solvent effect evaluated in acetonitrile by the IPCM (isodensity polarizable continuum model) method raises activation energies, Delta G(not equal), by 9 similar to 13 kcal mol(-1), but the relative reactivity order in the gas phase, -Delta G(not equal)(X,Y):(Cl, Br) > (Cl, Cl) > (Br, Br) > (Br, Cl), is maintained in solution. The stepwise mechanism predicted in the gas phase and in solution is consistent with the experimental result.