Dimer radical cations of methyl chloride have been prepared in a Fourier transform ion cyclotron resonance mass spectrometer using a consecutive exchange reaction of the xenon dimer radical cation with methyl chloride. Both theory and experiment have indicated the existence of two different stable methyl chloride dimer radical cations, that is a two-center three-electron (2c/3e) bonded dimer radical cation, CH3Cl therefore ClCH3.+, with C-2 symmetry and a methyl chloride chloronium ylidion structure, CH3Cl . H . ClCH2.+, with C-s symmetry. The CH3(C)l therefore ClCH3.+ species has been observed to react both as an electron acceptor and as a proton donor. The CH3Cl . H . ClC(H)3(.+) structure, however, reacts only as an electron acceptor. Calculations of the CH3Cl therefore ClCH3.+ structure at the MP2/6-311G(d,p) level of theory indicate a bond dissociation energy of 105.4 kJ mol(-1) relative to its ionized and neutral monomer products, while experiments show a bond dissociation energy of 109 +/- 5 kJ mol(-1). The bond dissociation energy of the CH3Cl . HClCH2.+ species has been determined to be 62.8 kT mol(-1) at the MP2/6-311G(d,p) level of theory, whereas experiments indicate 76 kT mol(-1) relative to the radical cation of methyl chloride and its neutral counterpart as products. These results indicate that CH3Cl therefore ClCH3.+ is the intermediate complex in the electron transfer reaction and CH3Cl . H . ClCH2.+ the intermediate complex in the proton transfer reaction between the methyl chloride radical cation and neutral methyl chloride.