The reversible intramolecular hydrogen transfer reaction of peptide Cys thiyl radicals with Gly and Ala residues was studied in model peptides, where thiyl radicals were either generated through photochemical cleavage of disulfide bonds or through the reaction of Cys thiol with (CH3)-C-center dot or (CH3CO)-O-center dot radicals, or both, generated through photolysis of acetone. In D2O, the reversible hydrogen transfer leads to covalent H/D exchange, indicative of the location of intermediary carbon-centered radicals. In addition, the reversible formation of C-alpha(center dot) radicals on Ala leads to the conversion of L-Ala to D-Ala, where the efficiency of this conversion depends on the primary sequence of the Ala-containing peptide. When Cys thiyl radicals are generated through the reaction of Cys thiol with (CH3)-C-center dot or (CH3CO)-O-center dot radicals, various recombination products between these initiating radicals and peptide thiyl and carbon-centered radicals provide further evidence for the location of intermediary radicals within the peptide sequence.