Abstract The interaction of thiols with unsaturated hydrocarbons was first reported by Posner, who treated thiophenol and benzylthiol with a variety of olefins at room temperature in the presence of acetic and sulfuric acids. With simple olefins, conjugated and nonconjugated, addition of fragments of the thiol produced by scission of the S-H link proceeded readily and, at asymmetrically substituted double bonds, in opposition to Markovnikov's rule: CR′R″:CHR‴+SHPh→CHR′R″⋅CH(SPh)R‴. More complex nonconjugated olefins (terpinolene and sylvestrene) showed depressed reactivity in that addition occurred at only one double bond, and the highly conjugated systems, stilbene and diphenylbutadiene, failed to react at all. Ashworth and Burkhardt confirmed Posner's formulation of the styrene-thiophenol reaction, and pointed out the catalytic activity of sunlight and the inhibition induced by small amounts of piperidine. Ipatieff and others investigated the reaction of thiophenol and ethanethiol and propylene, isobutylene, trimethylethylene, and isopropylethylene at 100–150°, and found that, although addition products abnormal with respect to Markovnikov's rule were formed in the absence of catalysts, yet the presence of sulfuric acid, contrary to Posner's findings, reversed the orientation of the addition and led to normal products. Contemporaneously, Jones and Reid and Kharasch, Reid, and Mayo showed that traces of peroxides, either as present normally in unsaturated hydrocarbons or as added ascaridole, strongly catalyze the abnormal addition, and that quinol effectively impedes the reaction. Mayo and Walling have proposed a free-radical chain-reaction mechanism, but kinetic evidence on which to found a detailed reaction scheme is so far lacking.