Palladium() complexes of peptides including Gly-Gly, Gly-Ala, Ala-Gly, Gly-Phe, Phe-Gly, triglycine, Gly-Gly-Ala and tetraglycine were studied by potentiometric and NMR spectroscopic methods. It was found that the reaction of free palladium() ion with dipeptides is almost complete below pH 2, therefore pH-metry can not be directly used for stability constant determinations. High excess of chloride ions (0.1 mol dm Ϫ3 ) was used to shift complex formation between [PdCl 4 ] 2Ϫ and peptides into the measurable pH range. The formation of the speciesϪ ) co-ordination was detected with all dipeptides in equimolar solutions. Bis complexes were formed in the presence of an excess of ligand, but their stoichiometry varied as a function of the amino acid sequences of the peptides. In the case of dipeptides with C-terminal Gly residues (X-Gly) the species [PdL 2 H -2 ] 2Ϫ with 4N co-ordination of two bidentate peptide molecules was formed by the physiological pH, while the stoichiometry of the bis complexes of dipeptides with non-co-ordinating side chains at the C termini (Gly-X) corresponds to [PdL 2 H Ϫ1 ] Ϫ containing both tridentate and monodentate peptide ligands. Equimolar solutions of palladium() and tripeptides are characterized by the formation of the species [PdLH Ϫ2 ] Ϫ with (NH 2 ,N Ϫ ,N Ϫ ,CO 2 Ϫ ) co-ordination, while [PdL 2 H Ϫ2 ] 2Ϫ containing two bidentate ligands is the main species in the presence of an excess of tripeptides.