A mass-selected, ion mobility drift-cell technique has been used to study the isomerization of the benzene radical cation generated by electron impact (EI) ionization. Evidence is presented for the generation of both the benzene and fulvene cations, with a lower limit for the barrier of isomerization E-a(benzene(+.) --> fulvene(+.)) estimated as E-a > 1.6 eV. The reduced mobilities of the benzene and fulvene cations are nearly similar, making it difficult to separate the two ions in pure helium. However, because of the large difference in the bonding of the benzene and fulvene cations with neutral benzene, the two isomeric ions are readily separated in the presence of neutral benzene in the drift cell. This suggests that isomer ions with similar mobilities can be separated on the basis of the degree of their associations with their neutral precursor in the drift cell. This separation concept allows us to discover the "hidden isomerization" process responsible for the generation of the fulvene cations via EI ionization of benzene. This also allows us to measure the equilibrium constant for the formation of the benzene dimer cation without interference from the fulvene isomer. The resulting binding energies (17.6 and 17.4 kcal/mol for (C6H6)(2)(+.) and (C6D6)(2)(+), respectively) are smaller than that reported by Hiraoka and co-workers (J. Chem. Phys. 1991, 95, 8413) and similar to that measured by Meot-Ner et al. (J. Am. Chem. Soc. 1978, 100, 5466), both using pulsed high-pressure mass spectrometry. The binding energies of the proton-bound dimers in pyridine and triethylamine systems have been measured as 25.2 +/- 1 and 20.9 +/- 1 kcal/mol, respectively, and the binding energies of the protonated methanol clusters H+(CH3OH)(n) with n = 3-5 have been measured as 21.0, 14.3, and 11.3 kcal/mol, respectively, in good agreement with literature values. The combination of thermochemical properties with isomer identification provides valuable information on the structure-property relationship of molecular cluster ions. The novel concept of the separation of isomeric ions by dimer formation is expected to be of general application and may help to resolve important outstanding issues in the separation, structure, and chemistry of hydrocarbon radical cations.