Cyclic voltammograms of pi-conjugated oligoalkylthiophene (13T) films are found to show two oxidation peaks during the 1st potential cycling up to 0.8 V. In the subsequent cycles, however, the 2nd oxidation peak disappears completely and the 1st peak shifts by ca. 0.1V to the cathodic direction. This finding along with spectroscopic measurements of 13T film before and after electrochemical stimulus suggests first that two different phases to be oxidized at separate potentials coexist in a pristine 13T film and second that one phase can be switched to the other by being oxidized to generate bipolarons.