Surface plasmon resonance (SPR) measurements were combined with cyclic voltammetry (CV) to analyze the electrochemical reactions of Fe(CN) 3-/4-6 on gold electrodes in the phosphate electrolyte. The time differential of the SPR signal was correlated with the current. When a low concentration of Fe(CN) 3-/4-6 was introduced, the replacement of adsorbed phosphate ions with Fe(CN) 3-/4-6 was observed in SPR. At higher concentrations, the SPR signal reflected the refractive index difference of the redox state of Fe(CN) 3-/4-6. After continuous potential scanning, the potential dependence of the SPR signal changed completely. However, the cyclic voltammogram was almost identical to that for the first cycle. The change in the SPR signal reflected the formation of a surface film, but electrochemical data showed that this film did not affect the electron transfer reaction in the experimental time window.