Binuclear, hydroxo-bridged, copper(II) complexes of a series of quadridentate pyridazine and phthalazine thioether ligands involving nitrogen donor groups (derived from pyridine, imidazole, benzimidazole) exhibit room-temperature magnetic moments in the range 1.1–1.7 B.M., indicative of antiferromagnetically coupled binuclear copper(II) centres. Cyclic voltammetry and coulometry on most of these systems indicate reversible or quasi-reversible redox processes, involving two-electron transfer, at positive potentials (0.23–0.49 V vs. a saturated calomel electrode in CH3CN or dimethylformamide). Mononuclear copper(II) derivatives, involving bidentate ligands, also exhibit reduction at positive potentials. Catecholase activity involving 3,5-di-t-butylbenzene1,2-diol has been demonstrated for the hydroxo-bridged complex [Cu2(ptpd)(OH)Cl3]·EtOH [ptpd = 3,6-di(2′-pyridylthio)pyridazine], in which a Michaelis-Menten kinetic treatment gave KM= 3.4 × 10–4 mol dm–3 and a value of kp= 2.1 × 10–2 s–1 for the dissociation of the catechol–complex intermediate.