Potentiometry and spectrophotometry were used to study the cobalt(II) complexes of L-cysteine, D-penicillamine, DL-mercaptosuccinic acid, N-acetyl-L-cysteine, N-acetyl-D-penicillamine, α-mercaptopropionylglycine and glutathione. It was found that L-cysteine and D-penicillamine form octahedral bis complexes of composition CoA 2 with the cobalt(II) ion. At intermediate pH values polynuclear complexes too are formed; this is the case primarily for L-cysteine. Pentacoordinated complexes CoA 2 are produced with mercaptosuccinic acid, and the concentration of the polynuclear species is negligible. N-acetyl-D-penicillamine forms a distorted tetrahedral bis complex by (S,O) coordination. Besides an analogous species, polynuclear and mixed hydroxo complexes too may be present in the cobalt(II)N-acetyl-L-cysteine system. In a weakly basic medium the cobalt(II) ion does not induce deprotonation of the peptide NH group for either mercaptopropionylglycine or glutathione. In the case of mercaptopropionylglycine the formation of the parent complexes is accompanied by their hydrolysis, while for glutathione there is the possibility of both cysteine-type and glutamic acid side-chain coordination.