2-(Toluenestdfanyl)-1 and 2-(benzenesuifanyl)-phenylac~ 10 reacted with benzenethiol and toluenethiol, respectively, in the presence of AIBN at 84 and 154 ยฐC to give prodm:ts deriving from vinyl radicals 2 which undergo hydrogen abstraction reaction and 5-orgm and 5-.exo eyclization onto both the adjacent phenyl rings in competition with the I~fragmentation. Definitive evidence has been obtained that alkenesulfanyl radical addition to the alkyne triple bond is a non-revere'hie process, whereas arenesulfa~ radicals add in a reveru%le mode. Competing expefime~ involving several alkynes towards benzenethiol and benzeneethanet~ol radical addition have been performed in order to determine the relative rate constaras of the suifanyl radical addition to the alkyne triple bond (k0 and the hydrogen abstraction reaction by the resulting vinyl radicals (kn). The ks values are mainly determined by the vinyl radical stabilization provided by the C-(~) vinyl radical mbstitue~ whereas the ks values seem to he mainly determined by polarity factors. An unexpected different behavior between ~t-propyl and a-long-chain substRuents is discussed in terms of different hybridization of the vinyl radical.