The electrochemical polymerization-depolymerization of an organodisulfide compound, 2,5-dimercapt-1,3,4-thiadiazole (DMcT), was investigated under in situ conditions by quartz crystal microbalance (EQCM) and atomic force microscopy (AFM) measurements employed simultaneously with electrochemical voltammetry. The EQCM results, together with the cyclic voltammetric results, suggest that the polymerization and depolymerization processes of DMcT do not occur with the same coulombic efficiency. The number of electrons consumed during the polymerization process is greater than that for the depolymerization process, indicating that some other chemical process occurs at the same time. The EQCM analysis and AFM observations indicated that the oxidation of DMcT, especially at higher potentials, involves not only a straightforward polymerization process via S-S bonds but also an overoxidation or degradation (decomposition) process of the poly(DMcT) deposited on the electrode surface. The AFM images of the deposited poly(DMcT) show smooth/uniform and isolated/island growth at low (0.8 V) and high (1.4 V) potentials, respectively. Substantial decomposition of poly(DMcT) was confirmed from both EQCM and AFM analysis when the polymer was formed at high potential (1.4 V). This behavior can be regarded as a serious drawback when the material is used as battery material.