Photochemical addition of toluene and o-, m-, and p-xylenes to cyclopentene gave principally 1,3 adducts (3,7-e/endo-tetracyclo[6.3.0.02,11.03′7]undecene-9) in which a methyl group was oriented specifically in C-l position. Addition of mesitylene to cyclopentene was exceptional in that both the 1,9,11- and 2,8,10-trimethyl derivatives were formed although the former predominated. From quantum yields for these additions, it was concluded that a methyl substituent on a benzene ring exercised a significant activation in the excited state on the ortho positions and possibly a deactivating effect on the meta positions. With increasing substitution of the aromatic ring with methyls, steric effects also came into play, the quantum yield for 1,3 addition for mesitylene being only ~0.1 of the value for benzene. Addition of benzene to 1,2-dimethylcyclobutene was as efficient as to cyclobutene. Addition of toluene to 1,2-dimethylcyclobutene was sixfold slower than to cyclobutene or m-3,4-dimethylcyclobutene. At the same time, the orientational specificity was retained in all these additions. A model of the complex between the singlet excited benzene and olefins is proposed which takes into account the features of the 1,3 additions. © 1973, American Chemical Society. All rights reserved.