The redox reaction of freshly prepared and well cycled Prussian blue (PB) films has been studied by means of an electrochemical quartz crystal microbalance (EQCM) and in situ Fourier transform infrared (FTIR) reflection spectroscopy. In the first reduction, insoluble PB (Fe-4(3+)[Fe-11(CN)(6)](3). 6H(2)O) was found to be converted irreversibly to Everitt's salt (ES, K2Fe2+[Fe-11(CN)(6)]) by incorporating 6 mol of potassium ions, and 6 mol of water molecules coordinated to iron ions are released simultaneously. The change in molar mass estimated from this reaction was the gain of 17.7 g mol(-1) per mol electrons, which was close to the observed value. The reoxidation of ES results in the formation of soluble PB (KFe3+[Fe-11(CN)(6)]) and the concurrent release of potassium ions, and water molecules are introduced to interstitial positions in the PB lattice. The reversible change between soluble PB and ES takes place in further repeated potential cycling.