Reactions of two platinum(II) complexes [Pt(dien)Cl]+ and [Pt(Gly-Met-S,N,N´)Cl], in which dien is diethylenetriamine and Gly-Met is the dipeptide glycyl-L-methionine coordinated through the sulfur and two nitrogen atoms, with N-acetylated dipeptide L-methionyl-L-histidine (MeCO-Met-His) have been studied by 1H n.m.r. spectroscopy. All reactions were carried out in 50 mM phosphate buffer at pD 7.4 and at room temperature. In the initial stages of the reactions both platinum(II) complexes form a kinetically favoured platinum–peptide complex with unidentate coordination of MeCO-Met-His through the sulfur atom of the methionine residue. In the second stages of the reaction an intramolecular migration of a [Pt(dien)]2+ unit from the sulfur to the nitrogen atom of imidazole has been observed. This migration reaction is very slow and strongly selective to the N 1 atom of the imidazole ring of the histidine side chain. No migration of the platinum(II) complex was observed in the reaction between [Pt(Gly-Met-S,N,N´)Cl] and the dipeptide MeCO-Met-His. It was found that the latter complex, with a more sterically hindered Gly-Met ligand, reacts more slowly with thioether-containing molecules than [Pt(dien)Cl]+ and forms a more stable platinum–sulfur bond. This study is an important step in the development of new platinum(II) complexes for selective covalent modification of peptides and proteins.