ABSTRACT:The addition reaction mechanism of thiophenol to ethynylbenzene was studied as a model of the polyaddition of 1,4-benzenedithiol to 1,4-diethynylbenzene. The addition reaction occurred without appreciable side reactions and an adduct of the anti-Markownikoff's structure ( q.,SCH = CHq.,) was quantitatively obtained. It was found that the addition reaction is very similar to the polyaddition of 1,4-benzenedithiol to 1,4-diethynylbenzene from the point of the elementary reaction. The kinetic study shows that the rate-determining step of the addition reaction is the addition reaction of the thiyl radical to the ethynyl bond. The results are compared with those of the addition reaction of thiophenol to styrene.