Abstract Both C 6H 6 + ions produced from benzene and C 7H 8 + ions from toluene contain isomers with different abilities to form dimers. Correspondingly, the mobilities of the isomers that are coupled reactively to dimers differ from the mobilities of the unreactive isomers. This allows separating the isomers and measuring the dimer binding energies of pure isomers. Equilibrium measurements determined the thermochemistry of (toluene) 2 + and (benzene) 2 + as Δ H 0 D=17.8±0.8 and 17.6 ± 0.9 kcal/mol, and Δ S 0 D=30.4±0.9 and 27.1 ± 0.7 cal/mol K, respectively. Ab initio calculations yield two structures for the (benzene) 2 +; a charge transfer parallel ‘sandwich’ dimer with a bonding energy of 17.6 kcal/mol, and a higher energy hydrogen bonded axial structure with a bonding energy of 15.2 kcal/mol.