Reduction processes of UO22+ in weakly acidic solutions were investigated based on electrochemical and spectrophotometric measurements. A reversible one-electron reduction of UO22+ to UO2+ and a further irreversible reduction of UO2+ were observed voltammetrically at a gold microdisk electrode in weakly acidic perchlorate solutions of pH from 2.0 to 3.5. Aggregates of U(IV) were formed as a deposit on the electrode and a colloid in the bulk solution, when the controlled-potential bulk electrolysis was carried out at a gold gauze electrode, even though the potential applied was that available for the first one-electron reduction wave of UO22+ observed at a gold microdisk electrode. It was elucidated that the aggregate was produced by the combination of the one-electron reduction to UO2+ and the disproportionation of UO2+ producing U(IV) and UO22+. The aggregate enhanced the rate of the disproportionation of UO2+, and hence the reduction current of UO22+ increased abruptly when a definite amount of aggregate was formed on the electrode, in the solution, or both. The aggregate of a two-electron reduction product of UO22+ was determined to be poorly crystallized UO2 of fluorite-type structure, and the characteristics of the aggregate were different from the crystalline UO2 (C) 2014 Elsevier Ltd. All rights reserved.