Cyclic and chronoamperometric electrochemical quartz crystal microbalance measurements are reported for polyaniline deposited on Au and immersed in aqueous solution of 1 mol dm−3 H2SO4. The results of piezoelectric microgravimetry indicate a mixed cationic and anionic ion transport. Although most of the frequency change after application of a potential step occurs within 1 s — attesting that even the anion transport is fast — a slow, logarithmic type relaxation can also be observed. The duration of the continuous mass change may be longer than 30 min, especially during a transition from reduced state to a medium oxidized state or vice versa. The change of the film resistance measured in situ shows a similar behaviour. The marked hysteresis effect between oxidation and reduction sequences may be explained a fast stabilization process including the planarization of the polymer and intermolecular coupling at higher charging level as well as a slower transformation at lower degree of oxidation. The effect of temperature is very small on the processes responsible for the relaxation phenomena and the hysteresis effect, consequently the enhancement of the chain motion may play a minor role, only.