Reactions of radical cations of eight stilbene derivatives (S-.+) have been studied using pulse radiolysis and gamma-ray radiolysis in 1,2-dichloroethane or butyl chloride. Unimolecular isomerization from cis-S-.+ to trans-S-.+ and bimolecular reactions with O-2 (oxidation) and a neutral stilbene (dimerization) occur depending on the substituents. The unimolecular c-t isomerization and the oxidation proceed preferably in S-.+ substituted with a p-methoxyl group (as an electron-donating substituent) with rate constants of k(i) = 4.5 x 10(6) to 1.4 x 10(7) s(-1) and k(02) = (1.2-4.5) x 10(7) M(-1) s(-1), respectively. On the basis of transient absorption measurements, it is concluded that separation and localization of a positive charge and an unpaired electron play the most important role as the controlling factors in the reactivities of the unimolecular isomerization and the oxidation. The dimerization involves initial formation of a pi-complex with overlapping of two benzene rings and is inhibited by steric hindrance of substituents on the benzene rings and olefinic carbons.