Various gas-phase C 8 H 1 0 radical cations were investigated by using tandem mass spectrometry (MS/MS) and by utilizing AM1, PM3 and MNDO semi-empirical computations. The dissociation properties of the radical cations of bicyclo[2.2.2]octa-2,4-diene (dihydrobarrelene, DHB), bicyclo[4.2.0]octa-2,4-diene (BCO), 1,3,5-cyclooctatriene (COT), and 1,3,5,7-octatetraene (OT) were found to change as a function of internal energy. Both MS/MS and computational results confirm that the DHB radical cation isomerizes to the BCO radical cation and to a lesser extent to the COT radical cation. The OTE radical cation does not interconvert with the BCO, COT or DHB radical cations. An attempt was made to generate a benzene/ethylene C 8 H 1 0 radical cation adduct ion via gas-phase ion/molecule reaction, but not adduct ion was observed in a chemical ionization source.