Ascorbate is an important water-soluble antioxidant, which when oxidized by reactive oxygen species is converted into dehydroascorbate (DHA). If not rapidly reduced back to ascorbate, DHA decomposes to a reactive 5-carbon compound (DHA*, +130 Da) that can modify reduced cysteinyl residues in peptides and proteins in vitro. The formation of cysteine adducts by DHA* was characterized by mass spectrometry using reduced insulin B-chain, α-lactalbumin, and hemoglobin. Mass spectrometry of DHA* modified insulin B-chain revealed the presence of one and two DHA* adducts. Enzymatic cleavage and tandem mass spectrometry of modified peptides allowed unambiguous localization of DHA* to the two cysteine residues in positions 7 and 19 of the insulin B-chain. Incubations of DHA with α-lactalbumin revealed that approximately 25% of the protein population was in a reduced state and could be modified by DHA*. The adduct was assigned to the N-terminally located cysteinyl residue in position 6. Incubation of hemoglobin with DHA followed by pepsin digestion and electrospray ionization tandem mass spectrometry (ESI-MSMS) of the peptide mixture allowed for the identification of three modified peptides. Tandem mass spectrometry of the modified peptides, two from the hemoglobin A-chain with identical mass and one from the hemoglobin B-chain, gave a complete series of y-type fragment ions, which were assigned to the cysteine containing peptides (100)LLSHCL(105) (A-chain), (101)LSHCLL(106) (A-chain), and (111)VCVLAHHFGKE(121) (B-chain). Although the DHA* adduct was lost from the peptides derived from α-lactalbumin and hemoglobin before fragmentation of the peptide bond, carbamidomethylation of the proteins prior to incubation with DHA abolished the formation of DHA*-protein adducts and confirmed that the target was indeed the cysteine thiol group. Future studies are focused on the modification of proteins by DHA* in cells and in vivo.