The deposition efficiency, porosity and redox behavior of Prussian blue (PB) thin films are investigated. To find better deposition conditions, the PB thin films were deposited on a Pt electrode using a potentiostatic method proposed by Mortimer and Rosseinsky (J. Electroanal. Chem. 151 (1983) 133). The optimal deposition potential was determined to be 0.50 V (vs. Ag|AgCl) by considering the deposition efficiency and the time needed for the deposition. The mass change on the Pt electrode was monitored using an electrochemical quartz crystal microbalance (EQCM) during the PB deposition. The deposition efficiency was calculated to be about 38% based on the charge consumption of the PB thin film during the reduction at optimal deposition conditions. The porosity of the PB thin film was estimated to be 0.39 for a PB film with a thickness of 1860 Å. During the redox reactions, cations in the electrolytic solutions must be inserted into or extracted out of the PB thin film. The lattice structure of the PB thin film limits its ease of cation insertion. It was found that K+ can move in and out of the PB lattices much more easily than Li+ because the hydrated radii of the K+ ions are smaller than those of the Li+ ions. Nevertheless, reduction of the PB thin film can be carried out partially and irreversibly in solutions containing Li+ or H3O+ alone. However, a mass loss for the thin film was observed.