We clarified the chemical reaction of deposits following the reduction of uranyl ions (U^VIO₂²⁺) from the results of electrochemical quartz crystal microbalance, impedance spectra, and X-ray absorption fine structure measurements. We propose the following deposition mechanism: (1) U^IV is formed by the disproportionation of U^V, (2) U^IV forms U^IV hydroxide deposits, and (3) finally, the hydroxide deposits change to U^IV oxide, which generally have a larger electrical resistance than the hydroxide form.