The mercapto group of cysteine (Cys) is a predominant target for oxidative modification, where one-electron oxidation leads to the formation of Cys thiyl radicals, CysS(center dot). These Cys thiyl radicals enter 1,2- and 1,3-hydrogen transfer reactions, for which rate constants are reported in this paper. The products of these 1,2- and 1,3-hydrogen transfer reactions are carbon-centered radicals at position C-3 (alpha-mercaptoallcyl radicals) and C-2 (C-center dot(alpha) radicals) of Cys, respectively. Both processes can be monitored separately in Cys analogues such as cysteamine (CyaSH) and penicillamine (PenSH). At acidic pH, thiyl radicals from CyaSH permit only the 1,2-hydrogen transfer according to equilibrium 12, (+H3NCH2CH2S center dot) reversible arrow (+H3NCH2CH)-C-center dot-SH, where rate constants for forward and reverse reaction are k(12) approximate to 10(5) s(-1) and k(-12) approximate to 1.5 x 10(5)s(-1), respectively. In contrast, only the 1,3-hydrogen transfer is possible for thiyl radicals from PenSH according to equilibrium 14, (+H3N/CO2H)C-alpha-C(CH3)(2)-S-center dot reversible arrow (+H3N/CO2H)C-center dot(alpha)-C(CH3)(2)-SH, where rate constants for the forward and the reverse reaction are k(14) = 8 x 10(4) s(-1) and k(-14) = 1.4 X 10(6) s(-1). The C-center dot(alpha) radicals from PenSH and Cys have the additional opportunity for beta-elimination of HS center dot/S center dot-, which proceeds with k(39) approximate to (3 +/- 1) X 10(4) s(-1) from C-center dot(alpha) radicals from PenSH and k(-34) approximate to 5 x 10(3) s(-1) from C-center dot(alpha) radicals from Cys. The rate constants quantified for the 1,2- and 1,3-hydrogen transfer reactions can be used as a basis to calculate similar processes for Cys thiyl radicals in proteins, where hydrogen transfer reactions, followed by the addition of oxygen, may lead to the irreversible modification of target proteins.