Irradiation of solutions of benzene in cyclopentene at 254 nm led to the formation of cyclopentene dimers and two 1,3 photoadducts of benzene to cyclopentene. The major adduct (IV) was shown to have the structure 3,7-exoisomer of IV was shown to have the structure 3,7-endo-tetracyclo[6.3.0.02, 11.03,7]undecene-9 while the minor adduct (V) was the 3,7-exo isomer of IV. These structures which were based on spectroscopic evidence were confirmed by the hydration of the compounds to the corresponding alcohols, syn- and anti-2-hydroxytricyclo[6.3.0.03,7]undecene-4, the structures of which were established by nmr studies as well as by oxidation. Quantum yields for the 1,3 addition of benzene to cyclopentene in cyclohexane solution were obtained as a function of the concentration of the olefin. These results when compared to corresponding data for other olefins indicate that cyclopentene is one of the most reactive of olefins in this addition. A mechanism that involves the addition of the olefin directly to the singlet (B2u) state of benzene rather than an (isomeric) intermediate state derived from it would be consistent with these observations. © 1973, American Chemical Society. All rights reserved.