Three polythiophenes, the parent molecule (PT), poly(3)methylthiophene (PMT) and polybithiophene (PBT) were potentiodynamically overoxidized at potentials U-OO positive to the reversible doping/undoping redoxpeaks U-rev(+) up to 2.5 V vs see. The water concentration in the acetonitrile with 0.1 M QAS was varied in the range of 0.002 to 20 M. Large, irreversible overoxidation peaks are observed with peak potentials ranging from 1.8 to 2.2 V vs see (at 0.1 M H2O). Coulometric evaluation with respect to the reversible charge, which is characterized by y = 0.33, one charge per 3 monomer units, shows that a Y-factor of 2-14 is characteristic for the overoxidation process. At low water concentrations, up to 0.1 M H2O, Y corresponds to 4 F per 3 monomer units. It is attributed to the dioxide (sulphone) formation at one of those three monomer units. FTIR reveals in addition that the SO, group is even present just after the galvanostatic electropolymerization due to the close vicinity of the potentials U-rev(+), and U-OO. This partial overoxidation has nearly no influence on the electronic conductivity. At higher water concentrations, 1-10 M, a further oxidation proceeds with an overall charge stoichiometry of 12 F/mole. This corresponds to an oxidative SO, elimination, described previously for the monomer, and the formation of carbonyls in the 2-, 3- and 5-position. At the vicinal dicarbonyl, anodic C-C cleavage occurs finally, and two carboxylic groups arise, which is compatible to the result of a leaching with 0.01 M NaOH in 50% isopropanole. The results open the possibility for a systematic anodic modification of the conducting polymer through organic electrochemistry in the solid state. Transport processes are not limiting due to the porosity of the polymer layer. Swelling in MeCN seems to play some role in addition.