Electrochromism of Prussian blue (PB) involving proton insertion/extraction was investigated in an aqueous HCl solution using a Ta2O5 . nH(2)O/PB laminate, formed by spin-coating a peroxo-polytantalate solution on an electrodeposited PB film. Reversible and stable electrochromic performance was observed with more than 10000 repeated coloring and bleaching cycles, demonstrating that the film of Ta2O5 . nH(2)O serves as a proton-conductive protecting layer to prevent PB from dissolving in a strong acid solution. Insertion behavior of proton into the PB framework is compared with those for K+, Rb+ and Cs+ by means of a cyclic voltammogram and equilibrium potential measurements. The equilibrium potentials measured as a function of the degree of intercalation have shown that the maximum accommodation of proton is 3.0 (per Fe-4(3+)[Fe-II(CN)(6)](3) unit), which is in agreement with K+ and Rb+. However, the potential-composition curve observed with the PB-proton system is quite different from others not only in the potential range but also in shape, featuring by a conspicuous inflection at the middle region of the intercalation level. The intercalation behaviors are also discussed using a lattice statistical model.