Ions and solvent insertion/expulsion during the redox reaction of polypyrrole were investigated. Simultaneous measurement of impedance and mass/potential transfer function, thanks to a fast quartz crystal microbalance, allows species movement to be determined. For polypyrrole in CsCl, Na2SO4 and NaCl, cations and solvent were expelled and anions inserted when the film was oxidised. The insertion isotherm was obtained and it was shown that bigger ions are hosted in the polymer at cathodic potential.