The redox behavior of U(VI) adsorbed onto a phosphate-modified indium tin oxide (PM-ITO) electrode was investigated by cyclic voltammetry and was compared with that in an aqueous solution at pH 4.0. It was discovered that the reduction potential of U(VI) adsorbed onto the electrode to U(IV) was much more positive than that of UO22+ in the solution. This finding suggests that the adsorption of uranium onto the PM-ITO changes the equilibrium of uranium redox reaction.