1,3-Cyclopentanediyl radical cations 5(.+) were generated from tricyclo[3.3.0.0(2,4)]octanes (housanes) 5 through chemical oxidation with tris(4-bromophenyl)aminium hexachloroantimonate (TBA(.+)SbCl(6)(-)) and shown to afford the regioisomeric olefinic products 6 and 7 on methyl 1,2 migration. A complete reversal in the regioselectivity of the 1,2 shift was observed, which reflects the electronic character of the X substituent at the migration terminus in the radical cation 5(.+). The regioselectivity is rationalized in terms of a simple MO interaction diagram by considering the epsilon(SOMO) orbital energies (AM 1 method) of the X-substituted radical fragments in the intermediary 1,3 radical cations 5(.+) relative to that of the cumyl radical fragment. The excellent correlation between the calculated orbital energy differences (Delta epsilon) and the experimentally observed regioisomeric ratios allows a quantitative assessment of the electronic substituent effects. The diastereoselectivity of the 1,2 shift is controlled by the steric factors in the intermediary 1,3 radical cations 5(.+).