Solutions of norbornene in benzene, on irradiation at 253.7 nm, yielded the two dimers of norbornene reported previously6 as well as two 1,3 adducts of benzene to the olefin. These adducts were the only significant products when the reaction was carried out at high (∼90%) concentrations of norbornene. The quantum yield for the addition (to give either adduct) increased with increasing concentration, the values being 0.27 and 0.05 in 8 M solution. The ratio of the adducts seemed invariant with experimental conditions. The adducts are believed to be isomeric pentacyclo[8.2.1.02,9.03,6.04,8]tridec-6-enes. The minor adduct, since it is quantitatively transformed to the major adduct on heating to 280°, probably is the more strained 2,9-endo (with respect to the benzene), exo (with respect to the norbornene) isomer while the major adduct is the 2,9-exo, endo compound. Quantum yield data for the dimerization reaction were also obtained. The implication of the addition to the kinetics and stereochemistry of the sensitized dimerization is discussed.