Oxidized poly(2,5-di-(-2-thienyl)-pyrrole was synthesized electrochemically from a 5 to 20 mM monomeric solution in 0.1 M LiClO4 + acetonitrile (ACN). The electrochemical behaviour of films was studied in 0.1 M LiClO4 aqueous solution, where an insoluble reduced form was found. Solubilities of both oxidized and reduced states were determined in DMSO, acetone and ACN. Their densities and electrical conductivities were also obtained. The solubility of the reduced form is about 15 times higher in DMSO and ACN than that of the oxidized form. The oxidized polymer is insoluble in ACN solutions of perchlorate salts. The high solubility of the reduced state in this medium allows a quantitative electrodissolution of the polymer following the reduction process. Results from electrochemical experiments, ultramicrogravimetry and elemental analysis indicate an average presence of 0.65 ClO4- ions per monomeric unit incorporated in the oxidized polymer. Both states contain soluble oligomers with from two to six monomeric units, as detected by mass spectrometry (fast atom bombardment). The molecular mass of these oligomers indicates a loss of two protons per monomer incorporated in the polymer, in agreement with an electropolymerization mechanism involving polycondensation of radicals.