Guided ion beam tandem mass spectrometry techniques are used to measure the reaction cross sections of the collisionally activated process I- + CH3Y --> products, where Y = Cl and Br. The Cl- and Br- products are observed at the lowest collision energies. A back-side attack S(N)2 reaction is responsible for the initial rise from the threshold. At higher collision energies, the ICl- and IBr- ions are observed and signify competition from a front-side-attack, halogen-abstraction reaction. All the reactions are endoergic and exhibit excess threshold energies, E-0, when compared with established reaction endothermicities from the literature, DeltaH(0). The reaction mechanisms are explored with the aid of CCSD(T)/LanL2DZ and CCSD(T)/SDD molecular orbital calculations and phase space theory.