The formation, chemistry, and nature of the thiyl peroxyl radicals, RSOO(.), are investigated by ESR and UV-vis spectroscopy in a variety of organic and aqueous matrices. Experimental evidence suggests that the unusual properties of thiyl peroxyl radicals result from the specific nucleophilic interaction of solvent which stabilizes the charge transfer state, RS(+)OO(.-). Anisotropic oxygen-17 coupling constants derived from ESR spectra of O-17 labeled species which are proportional to the unpaired spin at the oxygen are used to estimate the spin density distribution in RSOO(.) radicals. The couplings and spin density distribution are found to vary with the nature of the thiol and the matrix. For example, in freon the thiyl peroxyl O-17 couplings for the terminal and inner oxygens ((17)O1, (17)O2) differ for primary (79, 62 G), secondary (84, 57 G), and tertiary alkyl thiols (96, 51 G), whereas in aqueous systems or methanol all thiols yield RSOO(.) radicals of approximately the same couplings (80, 62 G). All RSOO(.) species in polar media have a visible absorption (lambda(max) = ca. 540 nm) and are found to undergo photoisomerization to RSO(2)(.) and subsequent oxygen addition to form RSO(2)OO(.). About 5-10% of the spin density is found on the sulfur atom. Results found in neutral or acid aqueous glasses show no pH dependence of either the O-17 hyperfine couplings or visible absorption maximum. The degree of charge transfer and the varying oxygen couplings are suggested to be a function of the ability of the medium to act as an electron pair donor. For tertiary thiols solvent access is sterically hindered in freons, which results in O-17 couplings and spin distribution much like that found for a usual carbon-centered peroxyl radical. The solvent-stabilized charge transfer state, RS(+)OO(.-), is found to be far more thermally stable than the uncomplexed state, which is found to react likely by thermal isomerization to RSO(2)(.) at 100 K. Ab initio MO calculations are found to mimic the charge transfer state by association of negative ions such as OH- or F- with the sulfur atom.