High-Pressure Mass Spectrometric (HPMS) experiments have been carried out to probe the details of the double minimum potential energy surface for gas-phase S(N)2 reactions. The well depths and entropy changes associated with the formation of entrance and exit channel electrostatic complexes for the chloride and bromide adducts of methyl, ethyl, isopropyl, and tert-butyl chlorides and bromides have been determined from the temperature dependence of the equilibrium constants for adduct formation. In the cases of "symmetric" complexes associated with identity S(N)2 reactions, there is an increase in well depth as the size and, therefore, polarizability of the alkyl group increases. Concomitant with this is an increase in the magnitude of the negative entropy change for complex formation which is the result of an increase in the frequency of the intermolecular mode(s) of the complex arising from the increased bond strength. The data for the unsymmetrical adducts for the non-identity S(N)2 reactions show the same pattern of increasing well depth with increasing alkyl group size with the chloride adducts of alkyl bromides being more strongly bound than the bromide adducts of the corresponding alkyl chlorides. Enthalpies and entropies associated with transition state formation are determined from the temperature dependence of the rate constant for the net halide displacement reaction. These data show that the transition state for the reaction of chloride ion with alkyl bromides may lie below (CH3Br), near (C2H5Br), or above (i-C3H7Br, t-C(4)Hs(B)r) the energy of separated reactants. These three situations exhibit different changes in rate constant with increasing temperature. In addition, the lifetime of the transient, chemically activated intermediate formed between chloride ion and methyl chloride has been determined from the pressure dependence of the rate constant for formation of the observable, collisionally stabilized electrostatic adduct. The lifetime thus obtained is in excellent agreement with trajectory calculations performed by Hase and co-workers.