The potential energy surface along the gas-phase S(N)2 reaction coordinate for a series of substituted benzyl chlorides undergoing identity exchange with chloride ion, Cl- + X-C6H4CH2Cl, has been investigated using both experimental and theoretical methods. The rate of identity substitution and the equilibrium ion-molecule complexation energy (chloride affinity) for the series X = H, m-CH3, m-OCH3, m-F, m-Cl, and m-CF3 have been measured using Fourier transform ion cyclotron resonance (FT-ICR) spectrometry. Application of RRKM theory to the experimental kinetics yields estimates for the activation energies. AM1 semiempirical calculations1 were also used to estimate the energetics of the surface for comparison with experiment and to extend the results to other systems. We show that although the experimental observables vary over a considerable range, the intrinsic activation energy, as measured from the ion-molecule complex, remains constant for the entire series of substituted benzyl chlorides studied.