To get more insight into the reactions of nitrosoarenes with thiols which may be responsible for cytotoxic effects, the reaction mechanism was studied with nitrosobenzene and 1-thioglycerol as model compounds. A transient intermediate was isolated by high performance liquid chromatography and identified as 2,3-dihydroxypropanesulfenic N-hydroxyphenylamide ("semimercaptal") by UV, 13C-NMR, and FAB mass spectroscopy. In aqueous solution this labile compound reassembles into 2,3-dihydroxypropanesulfinic phenylamide in a first order reaction. In the presence of excess thiol or ascorbic acid the "semimercaptal" is reduced to 2,3-dihydroxypropanesulfenic phenylamide without transient formation of a complete "mercaptal". Hydrolysis rather than thiolysis liberates aniline from the sulfenic phenylamide. Both the sulfinic and sulfenic phenylamides were obtained in crystalline form and identified by NMR and FAB mass spectroscopy. A scheme is presented of the known reactions of nitrosoarenes with thiols.