The electrooxidation behavior of thiophene and 8-methylthiophene on a Pt surface in an acetonitrile + tetrabutylammonium tetrafluoroborate solution were investigated. The electropolymerization of these monomers was studied in neutral, acidic, and basic media. The effects of thiophene concentration and the added acid were elucidated. The polythiophene and poly(3-methylthiophene) films formed were characterized by their cyclic voltammograms in a blank solution and dry conductivities were measured. Electrochemical properties of these polymers in the same medium were investigated in the absence and in the presence of added anhydrous acid and base. The behavior of the freshly prepared films was compared with that which lost its electroactivity as a result of electrooxidation using cyclic voltammetry, controlled potential coulometry, and FTIR spectroscopy. The mechanisms related to the formation of the polymers and their electroactivity loss were proposed.