Potentiodynamic generation of polypyrrole films at an extremely low reaction rate causes the resulting material to show an additional oxidation wave that is more negative and sharper than the one normally observed. The additional peaklike oxidation wave belongs to an unknown structural entity PPy(II). The treatment of acetonitrile with basic alumina prevents its formation, whereas the addition of a low concentration of hydrochloric acid (10(-5) to 5 x 10(-5) M) to the acetonitrile electrolytic solution promotes its formation. However, higher concentrations of HCl (>10(-4) M) results in the disappearance of the characteristic peaks of both PPy(II) and the conventional PPy(I). Instead, following the acid-catalyzed trimerization of pyrrole, an ill-defined, structurally changeable, and partially conjugated PPy(III) from electrochemical side reactions may be deposited on the electrode. In a solution of high acidity, the application of a low potential to the electrolysis passivates the electrode, owing to the formation of nonconductive PPy(III) with a low degree of conjugation. A schematic mechanism for several possible pathways of pyrrole polymerization is proposed.