Mononuclear and binuclear copper(II) complexes of the potentially tetradentate binucleating ligands 1,4-di(3′-methyl-2′-pyridyl)aminophthalazine (PAP3Me) and 1,4-di(5′-methyl-2′-pyridyl)aminophthalazine (PAP5Me) are reported. In the case of the ligand 1,4-di(4′,6′-dimethyl-2′-pyridyl)aminophthalazine (PAP46DiMe) only binuclear derivatives were obtained. The mononuclear complexes, involving protonated ligand, result from the sensitivity of these ligands to mildly acidic conditions in aqueous solutions of certain copper salts. Binuclear, neutral ligand, complexes are also formed when the pH increases. Hydroxy bridged binuclear structures exist in all the binuclear complexes described with the exception of one acetate bridged derivative. Sub normal magnetic moments are observed for all the binuclear complexes (< 1.74 BM) indicating spin coupled systems with very low values (< 0.88 BM) for binuclear complexes of PAP46DiMe, indicating the possibility of strong antiferromagnetic exchange between the metal centres.