Abstract The reactivity of a series of potentially tetradentate hydrazone ligands, involving pyridyl and imidazolyl substituent groups, towards copper(II) salts has been examined. Both mononuclear and polynuclear derivatives are obtained with some ligands and in some cases redox reactions are observed in which, when water is a significant solvent component, nitrogen gas evolution indicates the formation of copper(I) derivatives. The reduction is assumed to occur by initial hydrolysis of the hydrazone ligand, forming hydrazine as one product, which reduces copper(II) to copper(I). However the copper(I) ions bind preferentially to unoxidized ligand thus limiting the extent to which reduction occurs. In the presence of electronegative ligands the copper(II) complexes are stabilized in some cases, while in one case a mixed valence polynuclear species is produced. Preliminary details of the X-ray structure of [Cu(IMAA)Br 2]·H 2O (IMAA = (1-methyl-2-imidazolyl)aldazine) indicate a mononuclear, five-coordinate, system involving unsymmetrical tridentate ligand, a structural feature which is apparent in most other mononuclear species.