The S(N)2 nucleophilic displacement reaction of chloride ion with isopropyl bromide (i-PrBr) has been studied in nitrogen buffer gas at a pressure of 640 Torr over the temperature range 20-175 degrees C by ion mobility spectrometry (IMS). It is concluded that, under these conditions of relatively high buffer gas pressure, this nucleophilic displacement reaction occurs primarily by the distinctly two-step mechanism Cl- + i-PrBr reversible arrow Cl-(i-PrBr) --> Br- + i-PrCl in which a thermal energy ion complex, Cl-(i-PrBr), is maintained in a state of chemical equilibrium with the reactants. An S(N)2 displacement reaction then occurs within the thermal energy cluster ion by its unimolecular conversion to products. Equilibrium constants, K-1, and rate constants, k(1), for Cl- + i-PrBr reversible arrow Cl-(i-PrBr) and Cl-(i-PrBr) --> Br- + i-PrCl, respectively, are determined from the IMS spectra as a function of temperature. In addition, second-order clustering to form the ion complex Cl-(i-PrBr)(2) is also observed, and equilibrium constants, K-2, for this process are also obtained from IMS spectra. By these measurements, the major features of the potential energy surface for this reaction are characterized and place its S(N)2 transition state at 1.6 kcal mol(-1) above the energy of the reactants.