Ion transport during the redox switching of a Prussian blue (PB) / polyaniline (PAn) bilayer electrode has been investigated by means of an electrochemical quartz crystal microbalance (EQCM) and in situ Fourier transform infrared (FTIR) reflection spectroscopy. The movement of ions in the potential cycling of the bilayer electrode is affected considerably by the thickness ratio of PB to PAn and the electrode potential. It is at the bilayer electrode with the medium thickness ratio (2.5-3.25) of PB (165 nm) to PAn that cation (K+) and anion (Cl-) move simultaneously in opposite directions. On the positive scan of such an electrode, K+ ions are first ejected from PB to the PAn layer into which Cl- ions are concurrently taken from solution to maintain the charge balance. The oxidation of PAn at more positive potential leads to the expulsion of Kf ions to solution and the simultaneous incorporation of Cl- ions.