Ion movement in the dedoping-doping process of polypyrroles in acetonitrile has been investigated by the electrochemical quartz crystal microbalance on selected polypyrroles with and without N-alkyl substitution. The dedoping process involves reversible anion explusion for the former but irreversible cation incorporation for the latter. Electrolyte uptake by the polymer promoted by the polar pyrrole moieties accounts for the behavior of the N-unsubstituted polypyrroles.