The thiyl radicals derived from 2-mercaptoethanol and thiophenol were found to undergo a complex series of reactions with 1,1,3,3-tetramethyl-2,3-dihydroisoindol-2-yloxyl (TMIO). Thus, treatment of 2-mercaptoethanol with di-t-butyl peroxyoxalate (DTBPO) in the presence of TMIO produced two N-S compounds-a sulfoxamide and a sufonamide-not the expected N-O-S adduct. The reaction between thiophenol and TMIO, which proceeded at a reasonable rate in the absence of DTBPO, produced 1,1,3,3-tetramethyl-2,3-dihydroisoindolin and its corresponding phenylsulfoxamide, diphenyl disulfide, phenylsulfinic acid, and 1,1,3,3-tetramethyl-2,3-dihydroisoindol-2-ylphenylsulfonate (the adduct of TMIO and the phenylsulfonyl radical). The mechanism of formation of these products, and the use of TMIO for trapping S-centred radicals, are discussed. A new radical fragmentation process, which appears to be general for aminoxyl adducts of electron-rich systems, is described.