The reactions of the reversible addition of thiols and thiyl radicals to the nitrone spin traps DMPO (5,5-dimethyl-1-pyrroline N-oxide) and DEPMPO (5-diethoxyphosphoryl-5-methyl-1-pyrroline N-oxide) are described. Addition of the thiols to the double C=N bond of the nitrones results in the formation of the corresponding hydroxylamines, measured using P-31 NMR and the phosphorus-containing trap DEPMPO. Subsequent mild oxidation of these hydroxylamines into the paramagnetic adducts may interfere with genuine spin trapping of thiyl radicals representing the Forrester-Hepburn mechanism. The reverse decomposition of hydroxylamines to the parent nitrone and thiol and of paramagnetic adducts to the nitrone and thiyl radical were observed for the first time. The recycling of reduced thiols from thiyl radicals by nitrones may comprise the mechanism of their effective antioxidant activity, in vivo. The release of thiyl radicals upon the breakdown of the paramagnetic adduct may significantly affect not only the quantitative analysis of the spin trapping data but even the conclusions regarding the origin of short-lived radical intermediates. The equilibrium constant for the reactions of the formation of the product of DEPMPO with S-centered nucleophiles decreases in the series: sulfite > thioglycolic acid > cysteine > glutathione. The rate constants for the reaction of the monomolecular decomposition of the radical adducts back to the nitrone and glutathiyl radical were found to be equal to 0.3 +/-0.1 s(-1) and 0.02 s(-1) for DMPO/GS(.) and DEPMPO/GS(.) radical adducts, correspondingly.