Guided ion beam tandem mass spectrometry techniques are used to examine the competing chemical dynamics of reactions of fluoride ions with chloromethane in the center-of-mass collision energy range 0.05−30 eV. The exothermic bimolecular nucleophilic substitution (SN2) reaction F- + CH3Cl → CH3F + Cl- predominates at the lowest collision energies (0.05−0.1 eV) but decreases by a factor of ∼100 over the range 0.1−2 eV. Two endothermic product channels are detected at collision energies ∼1−20 eV, corresponding to proton transfer to form HF + CH2Cl- and chlorine abstraction to form CH3 + FCl-. The threshold energy for the proton-transfer reaction is E 0 = 97 ± 9 kJ/mol, which yields Δacid H 298(CH3Cl) ≤ 1653 ± 9 kJ/mol and EA0(CH2Cl) ≥ 0.77 ± 0.14 eV. The threshold energy for the chlorine abstraction reaction to form FCl- is E 0 = 170 ± 40 kJ/mol, which yields EA0(FCl) ≥ 2.6 ± 0.4 eV. Potential energy surfaces for the three reaction paths are calculated using the coupled cluster and density functional theory methods at the CCSD(T)/aug-cc-pVDZ and B3LYP/aug-cc-pVDZ levels. Geometry optimizations of stationary points along the surfaces show that a hydrogen-bonded F-···H−CH2Cl complex is 5 kJ/mol lower in energy than the C 3 v F-···CH3Cl complex. An additional feature observed in the Cl- reaction cross section at collision energies above 2 eV is attributed to further dissociation of CH2Cl- and FCl- products.