While the importance of thiols and their derivatives in biological processes is widely appreciated, the elucidation of the roles played by thiyl radicals in these processes - being hampered by the radical reactivity that makes their detection and characterization difficult - is lagging. The results of a spin-trap EPR study are reported which advance the capability for detecting and identifying thiyl radicals. Adducts with PBN (α-phenyl-N-t-butylnitrone) of thiyl radicals derived from the biologically abundant low-molecular-weight thiols cysteine, homocysteine, and glutathione are examined. Significant differences in the β-proton hyperfine couplings of the various adducts are observed; both the EPR lineshapes and the radical adduct lifetimes show trends reflective of the molecular size of the trapped thiyl radical. These results indicate that EPR spectroscopy can be useful in identifying specific thiyl radicals that may be involved in the biochemical reactions of low-molecular-weight thiols, protein thiols, and their derivatives. © 1992 Springer.