Reaction of potassium dichromate with gamma-glutamylcysteine, N-acetylcysteine, and cysteine in aqueous solution resulted in the formation of 1:1 complexes of Cr(VI) with the cysteinyl thiolate ligand. The brownish red Cr(VI)-amino acid/peptide complexes exhibited differential stability in aqueous solutions at 4 degrees C and ionic strength = 1.5 M, decreasing in stability in the order : gamma-glutamylcysteine > N-acetylcysteine > cysteine. H-1, C-13, and O-17 NMR studies showed that the amino acids act as monodentate ligands and bind to Cr(VI) through the cysteinyl thiolate group, forming RS-(CrO3-)-O-VI complexes. No evidence was obtained for involvement of any other possible ligating groups, e.g., amine or carboxylate, of the amino acid/peptide in binding to Cr(VI). EPR studies showed that chromium(V) species at g = 1.973-4 were formed upon reaction of potassium dichromate with gamma-glutamylcysteine and N-acetylcysteine. Reaction of potassium dichromate or sodium dichromate with N-acetylcysteine and the methyl ester of N-acetylcysteine in N,N-dimethylformamide (DMF) also led to the formation of RS-(CrO3-)-O-VI complexes as determined by UV/vis, IR, and H-1 NMR spectroscopy. Thus, an early step in the reaction of Cr(VI) with cysteine and cysteine derviatives in aqueous and DMF solutions involves the formation of RS-CrO3- complexes. The Cr(VI)-thiolate complexes are more stable in DMF than in aqueous solution, and their stability towards reduction in aqueous solution follows the order cysteine < N-acetylcysteine < gamma-glutamylcysteine < glutathione.