Thin films of polypyrrole (Ppy) were studied in aqueous electrolytes using electrochemical quartz microbalance (EQCM). The doping-dedoping process for active polymer polarised in aqueous electrolyte involves reversible simple anion expulsion. The evidence for cation motion was noticeable only for the polymer with reduced electrical activity. The state of the polymer depends on the size of the removed cation.