We clarified the chemical reaction of deposits following the reduction of uranyl ions (UVIO22+) from the results of electrochemical quartz crystal microbalance, impedance spectra, and X-ray absorption fine structure measurements. We propose the following deposition mechanism: (1) UIVis formed by the disproportionation of UV, (2) UIVforms UIVhydroxide deposits, and (3) finally, the hydroxide deposits change to UIVoxide, which generally have a larger electrical resistance than the hydroxide form.