Electrochemical oxidation of poly[(ethoxypropylsilanylene) penta(2,5-thienylene)] films is investigated in acetonitrile containing tetraethylammonium perchlorate. A cyclic voltammogram of the polymer film exhibits two pairs of redox waves with oxidation/reduction peak potentials at 0.64/0.46 and 0.92/0.77 V. Measurements of mass changes on the polymer electrode using a quartz crystal microbalance indicate that during the first redox process a doping/dedoping of perchlorate ions takes place in a reversible manner. The maximum doped charge in the potential range up to 0.7 V compares well with a theoretical value for the one-electron oxidation of the pentathiophene unit acting as an independent redox site. A spectroelectrochemical study reveals that two sorts of chemical species are generated in the first oxidation step: one is a cation radical (polaron) with absorption band maxima at 0.95 and 1.68 eV and the other a pi-dimer with band maxima at 1.23 and 2.05 eV. The pi-dimer becomes dominant as the applied potential increases. The second oxidation step, resulting in a dicationic state of the pentathiophene unit, is found to be accompanied by degradation of the polymer.