A novel bi-tetradentate polythioether ligand, 6,6-methylene-bis(5-mercapto-3-thiahexyl)-4,8-dithiaundecane-1,11-dithiol (H4L) was synthesized, and its di- and tetranuclear copper(II) complexes were prepared, and characterized by elemental analyses, magnetic moments, H-1-n.m.r., i.r., and Uv/vis spectra. The i.r. data show that the ligand acts in a tetradentate manner and coordinates via one S atom of the thioether and thiol groups. The geometry of the metal chelates is discussed with the help of magnetic and spectroscopic measurements. The elemental analyses, stoichiometry, and spectroscopic data of the complexes indicate that the copper(II) ions are coordinated to the bi-dianion of the ligand. The function of the thiol ligand is to release protons to form copper(II) complexes, (Cu2L).