We report unprecedentedly high electrocatalytic activity of electropolymerized films of poly(3,4-ethylenedioxythiophene) (PEDOT) toward the redox reactions of 2,5-dimercapto-1,3,4-thiadiazole (DMcT) which is a promising cathode material for high-energy Li-ion batteries. The irreversible redox reaction of DMcT monomer at an unmodified glassy carbon electrode was dramatically accelerated at a PEDOT film-coated electrode to give a reversible response. PEDOT also showed high electrocatalytic activity toward the redox reactions of DMcT dimer and oligomers. Such acceleration of the DMcT redox reactions, which is essential to practical application of organosulfur compounds in batteries, was ascribed to the electronic properties of PEDOT. Chronopotentiometric results indicated that at PEDOT film-coated electrodes the redox reactions of DMcT proceed fast enough to satisfy an electrolysis current as large as 1.0 mA/cm(2). (C) 2003 The Electrochemical Society.