The addition reaction between thiophenol and phenylacetylene has been studied.With molar ratio of 1:1 the two reactants add rapidly at 00, giving traps-f3-phenylmercaptostyrene in quantitative yield.When the addition reaction was conducted at -20° or -40°, the adducts consisted of about equal amounts of trans- and cis-β-phenylmercaptostyrene.The cis-β-phenylmercaptostyrene was converted to the traps isomer upon heating to 180°.Sodium thiophenolate added on phenylacetylene in absolute alcohol yielded cis-β-phenylmercaptostyrene as the sole product.