The electron transfer photoreaction between 9,10-dicyanoanthracene (DCA) and (E)-3,7-dimethylocta-2,6-dien-1-ol (geraniol, (E)-1) in dichloromethane produces mainly cis-2-(2-propenyl)-trans-5-methylcyclopentanemethanol, cis,trans-3, whereas irradiation of acetonitrile solutions of 1,4-dicyanobenzene (DCB) and (E)-1 forms mainly two trans-fused 3-oxabicyclo[3.3.0]octanes (4, R = H; 5, R = p-C6H4CN). Analogous results are observed in the photoreaction of farnesol ((E,E)-2) with DCB in the presence of phenanthrene as cosensitizer. The photoreaction of DCB with 1 or 2 has sufficient driving force (Delta G = -0.7 eV in acetonitrile) for the generation of solvent-separated radical ion pairs (SSRIPs); in contrast, the marginal driving force (Delta G approximate to 0 eV) of the DCA sensitized reaction allows only the formation of contact radical ion pairs (CRIPs). The resulting radical cations react by five-center C-C cyclization, yielding di-tertiary, 2,alpha-bifunctional methylidenecyclopentyl radical cations, cis- or trans-B-.+. Subsequently, CRIPs undergo rapid back electron transfer and intramolecular hydrogen transfer, generating product 3. The radical cations, trans-B-.+, of SSRIPs undergo a second cyclization by intramolecular nucleophilic capture, generating 3-oxabicyclo[3.3.0]oct-6-yl free radicals, trans-C-..