Reactions of palladium(II) complexes (cis-[PdCl 2 (en)], cis-[PdCl 2 (L-HMet-S,N)] and their aqua derivatives cis-[Pd(H 2 O) 2 (en)] 2 and cis-[Pd(H 2 O) 2 (L-HMet-S,N)] 2 , in which en is ethane-1,2-diamine and L-HMet is L-methionine coordinated through nitrogen and sulfur atoms) with L-alanyl-L-histidine (Ala-His), L-seryl-L-histidine (Ser-His), and the N-acetylated dipeptides MeCO-Ala-His and MeCO-Ser-His have been studied by 1 H NMR spectroscopy. In the reactions between palladium(II) complexes and Ala-His or Ser-His, the tridentate coordination of peptide occurs at 1.5 < pH < 3.5 and causes release of the en ligand and ring opening of the L-HMet chelate. The 1 H NMR data show that the peptides are bound to palladium(II) through imidazole N-3, deprotonated amide, and terminal amino nitrogen atoms. This tridentate coordination of the peptides is unfavourable for their hydrolysis.The reaction between palladium(II) complexes and the N-acetylated peptides MeCO-Ala-His and MeCO-Ser-His at 1.5 < pH < 3.5 proceeds without release of the en ligand or ring opening of the L-H Met chelate. In the reaction between the peptides and cis-[PdCl 2 (en)] or cis-[Pd(H 2 O) 2 (en)] 2 , ®ve NMR-detectable complexes are formed, whereas with cis-[PdCl 2 (L-HMet-S,N)] and cis-[Pd(H 2 O) 2 (L-HMet-S,N)] 2 only two palladium(II)-peptide complexes are observed in solution. No cleavage of the amide bonds in the reactions between the peptides and the palladium(II) aqua complexes occurs.