The electrochemical properties of the Pt electrodes modified with Prussian blue (PB), polyaniline (PAn) and inner PB\outer PAn films have been examined using in situ Fourier transform infrared spectroscopy. At the PB-modified electrode, two strong peaks were observed at 2040 and 2110 cm(-1) owing to the bridging CN ligand in the PB structure. Three absorption bands appeared at 2780, 1015, and 982 cm(-1) by stepping the potential to more positive than +0.2V vs. Ag\AgCl\1 M KCl, which were assigned to various vibrational modes of water molecules coordinated to the high-spin Fe3+ ion of PB. It is suggested that doping or undoping of potassium ions during the polarization of the Pt\PB electrode is accompanied by expulsion or introduction of water molecules respectively. At the PB\PAn-modified electrode, the absorption spectrum of PAn in the vibrational region was scarcely affected by the underlying PB. The spectral intensity of the PAn on Pt\PB was much larger than that of the PAn on Pt, which is brought about by the K+ ions being more effective as the charge-compensating ions in the PB\PAn film.