Irradiation of 1-phenylcyclopentene (1a) with 1,4-dicyanobenzene (DCNB) in acetonitrile gave [2 + 4] cycloadducts, tetralin-type dimers (2-4), together with a 2:1 adduct of 1a with DCNB (5), through an electron transfer reaction. However, similar irradiation of 1-phenylcyclohexene (1b) gave 1-cyano-2-phenylcyclohexane (7) and 1:1 adducts of 1b with DCNB (8 and 9), instead of yielding dimeric products. Transient absorption spectra obtained by pulse radiolysis and laser flash photolysis of 1a showed simultaneous formation of a new absorption band with lambda(max) around 480 nm, attributable to dimer cation radicals, and decreasing absorption bands for monomer cation radicals (1a(.+), lambda(max) : 400 and 670 nm). In the case of 1b, by contrast, only absorption bands for monomer cation radicals 1b(.+) were observed in a region similar to those of 1a(.+). The optimized structure of the monomer olefin cation radicals, calculated by the PM3 method, suggested that, unlike the chairlike structure of 1b(.+), the fully planar structure of 1a(.+) would, on interacting with another neutral 1a, promote the formation of dimer cation radicals.