Glutathione thiyl radicals (GS(•)) were generated in H(2)O and D(2)O by either exposure of GSH to AAPH, photoirradiation of GSH in the presence of acetone, or photoirradiation of GSSG. Detailed interpretation of the fragmentation pathways of deuterated GSH and GSH derivatives during mass spectrometry analysis allowed us to demonstrate that reversible intramolecular H-atom transfer reactions between GS(•) and C-H bonds at Cys[(α)C], Cys[(β)C], and Gly[(α)C] are possible.