Under well-controlled conditions, different polypyrrole variants can be galvanostatically prepared in acetonitrile + 1% H2O by changing the current density. At current densities down to 0.25 mA/cm(2), ppy(I) is electrosynthesized; at lower current densities, a mixture of PPy(I) and PPy(II) is generated. In the presence of a small amount of acid (similar to1 x 10(-5) M), PPy(II) is exclusively formed. The availability of well-defined materials enables us to perform further reliable characterization by EQCM. The results reveale different natures of ionic transport when different PPy variants are switched between oxidized and neutral states. When doped with PF6-, PPy(I) and PPy(III) show only anionic movement upon redox, whereas PPy(II) exhibits the transport of both anion and cation. The structural diversity of PPy explains some controversial results that were obtained in the past. The mechanistic analysis offers new insight into the formation paths of conducting polymers. Of particular interest is the electrochemical solid-state transition from PPy(II) to PPy(I) in pyrrole-free solution when a higher potential (> 1.4 V) is applied to the PPY(II)-coated electrode. Repetitive potential scans transform PPy(II) completely into PPy(I). The transition is clearly evidenced by voltammograms. EQCM demonstrated a consistent change in ionic movement. In situ conductivity measurements indicate different types of charge carriers generated during the charging of PPy(I) and PPy(II).