Prussian blue (PB), which is a mixed-valent polynuclear metal complex, was formed in the presence of the solid polymer electrolyte membrane Nafion (Nf). According to the stepwise dipping method, in which either Fe2+or Fe(CN)63−of the component ions of PB was first incorporated into the matrix membrane and then the membrane was immersed into a solution containing the other component ion, a thin layer of PB was deposited on the surface of Nf or inside Nf depending upon the sequence of incorporation of the component ions. A dissipative structure of periodic PB layer deposit inside Nf was found to be formed under a specific condition of the stepwise method. PB was also formed inside Nf as a broader line by the countercurrent diffusion method in which both component ions were incorporated into Nf from opposite surfaces. The position of PB in Nf varied with the concentration ratio of Fe2+to Fe(CN)63−in this case.