Electrochemistry of poly[(tetraethyldisilanylene)quinque(2,5-thienylene)] (DSST) has been investigated in acetonitrile with cyclic voltammetry, potentiostatic oxidation and spectroelectrochemical measurements. In multicycle voltammograms of a DS5T film there are a pair of redox peaks attributable to anion-doping and dedoping. During potential cycling or electrolyzing at certain positive potentials, the polymer is subject to decomposition to some extent. An irreversible oxidation subpeak for the decomposition of the polymer is observed in the first-cycle voltammogram with low scan rates. Spectroelectrochemical measurements demonstrate that the slow decomposition process tends to be inhibited by further oxidation of its decomposed product, followed by redepositing of a solid film onto the DS5T film. The further oxidation of the decomposed product also leads to some unexpected observations in the spectroelectrochemistry of DS5T. Moreover, if the forward potential scanning is returned at more positive potentials, two pairs of oxidation/reduction peaks occur in the cyclic voltammogram of DS5T and the polymer film becomes much more unstable at the potential of the second oxidation peak.