The systems of cobalt(II), nickel(II), copper(II), zinc(II), and iron(III) with L-prolyl-L-leucinehydroxamic acid [N-hydroxy-4-methyl-2-(pyrrolidine-2′-carboxamido)pentanamide](Pro-Leu-NHOH) and L-prolyl-L-leucylglycinehydroxamic acid [N-hydroxy-7-methyl-4-oxo-5-(pyrrolidine-2′-carboxamido)-3-azaoctanamide’(Pro-Leu-Gly-NHOH) have been studied at metal/ligand ratios of 1 : 1–1 : 6 by means of pH-metric, spectrophotometric, and e.s.r. methods. The formation constants have been determined and the bonding modes in the species present in aqueous solutions are discussed. Complexes of moderate stability are formed in the systems containing cobalt(II) and zinc(II) ions in the approximate range pH 6.0–8.5. There is no deprotonation of the peptide amides. Stable complexes are formed in the copper(II)–Pro-Leu-NHOH and –Pro-Leu-Gly-NHOH systems at above pH 4. It can be postulated that the hydroxamate nitrogen, peptide carbonyl oxygen, and terminal amino nitrogen take part in the co-ordination at the beginning of complex formation in the copper(II)–Pro-Leu-NHOH system, but the co-ordination is primarily ‘hydroxamate-like’ in the copper(II)–Pro-Leu-Gly-NHOH system in that region. However, deprotonation of the peptide amides takes place in both systems, after which only the nitrogen donor atoms (amino, amide, and hydroxamate) are involved in the co-ordination. Planar complexes involving co-ordination of the above nitrogen donor atoms predominate at above pH 6 in both systems containing nickel(II). In the iron(III)–containing systems complex formation is appreciable even below pH 3. Mixed hydroxo complexes can be proposed in addition to the binary ones. In these latter complexes, co-ordination of the hydroxamate oxygens is suggested.