Reduction and oxidation peak potentials of poly[dithio-2,5-(1,3,4-thiadiazole)] were observed in hot gamma-butyrolactam (90 degrees C) at -0.1 and 0.1 V vs, Ag respectively. To clarify the redox reaction of the polymer (oligomer), bis(2-methyl-1,3,4-thiadiazoyl)-5,5’-disulfane was synthesized as a model compound and its redox reaction examined by experiment and molecular orbital calculation. Reduction and oxidation peak potentials of this model were observed at -0.65 and 0.2 V respectively, potentials corresponding to the cleavage and formation reactions of the disulfide bond. The bond cleavage reaction was also suggested by molecular orbital calculations. From a comparison of the shape and response of the cyclic voltammogram between the monomeric and polymeric disulfides, it became clear that reduction and oxidation of the polymer meant the cleavage and formation reactions of the disulfide bond respectively, and that the redox reaction is quasi-reversible.