Density functional theory computations and pulsed-ionization high-pressure mass spectrometry experiments have been used to explore the potential energy surfaces for gas-phase S(N)2 reactions between halide ions and trifluoromethyl halides, X- + CF3Y -> Y- + CF3X. Structures of neutrals, ion-molecule complexes, and transition states show the possibility of two mechanisms: back- and front-side attack. From pulsed-ionization high-pressure mass spectrometry, enthalpy and entropy changes for the equilibrium clustering reactions for the formation of Cl-(BrCF3) (-16.5 +/- 0.2 kcal mol(-1) and -24.5 +/- 1 cal mol(-1) K-1), Cl-(ICF3) (-23.6 +/- 0.2 kcal mol(-1)), and Br-(BrCF3) (-13.9 +/- 0.2 kcal mol(-1) and -22.2 +/- 1 cal mol(-1) K-1) have been determined. These are in good to excellent agreement with computations at the B3LYP/6-311+G(3df)//B3LYP/ 6-311+G(d) level of theory. It is shown that complex formation takes place by a front-side attack complex, while the lowest energy SN2 reaction proceeds through a back-side attack transition state. This latter mechanism involves a potential energy profile which closely resembles a condensed phase SN2 reaction energy profile. It is also shown that the Cl- + CF3Br -> Br- + CF3Cl S(N)2 reaction can be interpreted using Marcus theory, in which case the reaction is described as being initiated by electron transfer. A potential energy surface at the B3LYP/6-311+G(d) level of theory confirms that the F- + CF3Br -> Br- + CF4 S(N)2 reaction proceeds through a Walden inversion transition state.