We present a characterization of the redox behavior of organosulfur-based composite cathodes composed of poly[dithio-2,5-(1,3,4-thiadiazole)] (PDMcT), which is a polymer derived from 2,5-dimercapto-1,3,4-thiadiazole (DMcT), and poly(3,4-ethylenedioxythiophene) (PEDOT) in a carbonate-based mixed solvent containing 1.0M LiBF4. We have previously shown that PEDOT films, electrochemically generated at glassy carbon electrode surfaces, gave rise to a dramatic enhancement of the interfacial charge transfer kinetics of DMcT in solution. In a similar fashion, chemically prepared PEDOT films exhibited dramatic electrocatalytic activity towards the redox reactions of PDMcT in the composite cathodes. While the composite cathode exhibited a very high capacity of 205mAhg−1 (based on the electroactive mass) at the first discharge, in subsequent charge/discharge tests, the capacity of the PDMcT–PEDOT composite cathode (1:1 mole ratio) decreased significantly because of dissolution of the reduction products of PDMcT into the electrolyte solution. We also found that an ionic polymer, consisting of a mixture of PEDOT and polystyrene sulfonate (PEDOT–PSS) could electrostatically, but not physically, prevent, at least in part, leaching of the DMcT species into the electrolyte solution, thus improving the coulomb efficiency for the redox reactions of DMcT in a PDMcT–PEDOT composite film during charge/discharge cycles.