Mass spectrometry and anion photoelectron spectroscopy have been used to study the gas-phase SN2 reaction involving Br- and CH3I. The anion photoelectron spectra associated with the reaction intermediates of this SN2 reaction are presented. High-level CCSD(T) calculations have been utilised to investigate the reaction intermediates that may form as a result of the SN2 reaction along various different reaction pathways, including back-side attack and front-side attack. In addition, simulated vertical detachment energies of each reaction intermediate have been calculated to rationalise the photoelectron spectra.