Twelve ESR-active (and one inactive) copper(II) complexes of L-histidylglycine (HL) were characterized via their formation (micro)constants and ESR parameters obtained by two-dimensional ESR spectroscopic evaluation in aqueous solution. In strongly acidic media, the ligand is coordinated through its N-terminal donor groups: the complex [CuLH(2)](3+) involves monodentate imidazole binding, whereas [CuLH](2+) involves bidentate ligation through the amino and imidazole N atoms. This histamine-like bonding mode also predominates in the isomers of [CuL(2)], formed at ligand excess near pH 7: in the major 4N isomer, both ligands occupy two equatorial sites, while in the 3N isomer, the second dipeptide is coordinated equatorially by the amino and axially by the imidazole groups. At above pH 3-4, deprotonation of the peptide group also starts: in approximately 60% of the molecules of [CuL](+), the peptide group is deprotonated, while in the minor isomer histamine-like coordination occurs. At higher pH, the active dimer [Cu(2)L(2)H(-2)], the mixed hydroxo complexes (the inactive [Cu(2)L(2)H(-3)](-) and the active [CuLH(-2)](-)), and the bis complexes [CuL(2)H](+) and [CuL(2)H(-1)](-) all involve tridentate equatorial ligation of the backbone by the amino and deprotonated peptide N and the carboxylate O atoms. In the active dimer, the neutral imidazole groups form bridges between CuLH(-1) units. In [CuL(2)H](+), the second ligand is bound equatorially via its imidazole group; in [CuL(2)H(-1)](-), the L ligand occupies the fourth equatorial site and an axial site through its amino and imidazole N atoms, respectively.