The electrochemical polymerization of pyrrole and 3,4-ethylenedioxythiophene (EDT) in phosphate buffer solutions in the presence of poly(styrene sulfonate) (PSS) was investigated by means of cyclic voltammetry and chronoamperometry. From the measurements it was found that the electrochemical polymerization of pyrrole and EDT could be initiated at 600 mV and 800 mV respectively in the phosphate buffer. To evaluate the stability of polypyrrole + poly(styrene sulfonate) (PPy + PSS) and poly(3,4-ethylenedioxythiophene) + poly(styrene sulfonate) (PEDT + PSS) films, a constant potential was applied to films immersed in phosphate buffer solutions. PEDT + PSS was found to be much more stable than PPy + PSS under constant polarization conditions. After polarization for 16 h at 400 mV and pH 7.5 only 5% of the original electrochemical activity of PPy + PSS remained, whereas PEDT + PSS retained 89% of its original activity. The stability is strongly dependent on the magnitude of the polarization potential and the pH of the buffer solution.