Polythiophenes, which are electrically conducting polymers, are generally insoluble except for 3-substituted derivatives with long alkane chains. The electrochemical oxidation of poly(3-methylthiophene) in solution without the monomer caused dissolution of the film at potentials similar to the polymerization potentials until the film was removed completely from the electrode. This is due to oxidative degradation. The kinetics of the dissolution was examined by time-variations of film thickness as well as of the mass by means of EQCM. Dissolved species, which were estimated from UV-, FTIR-spectroscopy and GC-MS, were monomer derivatives with a thiophene ring. When the solution included the monomer of which concentrations were over 6 mM, the oxidation caused not only dissolution but also polymerization.