The cyclic voltammetry of benzene 1, naphthalene 2, biphenylene 3, and anthracene 4 fully annelated with bicyclo[2.2.2]octene showed that their cationic species have enhanced kinetic and thermodynamic stability as compared with the corresponding polymethyl analogues. Chemical one-electron oxidation of these neutral aromatic hydrocarbons led to facile isolation of their radical cation salts, which are persistent at ambient temperature even in air, and allowed X-ray crystallographic determination of their structures for the first time for the monomer radical cations of alkyl-substituted benzene, naphthalene, and biphenylene. The pi -systems of these radical cations are planar except for that of the naphthalene radical cation 2(.+). The structure of the benzene ring of 1(.+) precludes the observation of a static Jahn-Teller distortion in the crystal at -100 degreesC. For the radical cations of 2-4, the geometrical changes in pi -systems upon one-electron oxidation are consistent with what can be predicted from the orbital coefficients of the HOMO of the neutral molecules. Characteristic changes in the lengths of the a-bonds of the bicyclic framework involved in sigma-pi conjugation (C-C hyperconjugation) were also observed. Theoretical calculations, using ab initio molecular orbital (HF/6-31G*) and density functional (B3LYP/6-31G*) methods, have been used to interpret the experimental data.