The Prussian blue modified electrode prepared has been characterized by mcans of X-ray diffractionmethod as well as infrared spectra. The result definitely confirms that the electroactive blue film firmlyattached to Pt substrate by electrodeposition is really composed of PB, and that the specially preparedthin films on Pt possess the same crystal structure as that of the ordinary PB synthesized chemically. Avery detailed study of ion effect of supporting electrolyte strongly suggests that both reduction and oxi-dation processes of PB film are accompanied by the penetration of only cations, but not anions. Cationssuch as Na~+, as well as K~+, NH_4~+ and Cs~+ can penetrate the film, while Li~+, Ba~(2_+) and Ca~(2_+) cannot.The redox reactions of PB film has been expressed as follows: The Prussian blue modified electrode prepared has been characterized by mcans of X-ray diffraction pattern as well as infrared spectra. The result is confirmed that the electroactive blue film firmly attached to Pt substrate by electrodeposition is really composed of PB, and that of specially prepared thin film on Pt possess the same crystal structure as that of the ordinary PB synthesized chemically. Avery detailed of ion effect of supporting electrolyte strongly suggests that both both reduction and oxi-dation processes of PB film are accompanied by the penetration of only cations, but not anions. Cationssuch as Na ~ +, as well as K ~ +, NH_4 ~ + and Cs ~ + can penetrate the film, while Li ~ +, Ba ~ (2_ +) and Ca ~ (2 +) cannot.The redox reactions of PB film has been expressed as follows: