Abstract: Methods for reduction of hexavalent uranium to the trivalent state by the liquid-zinc-amalgan and mercury cathode electrolysis have been investigated in hydrochloric and sulfuric acid solutions. The percentages of reduction were determined with different media, various acid concentrations, uranium concentrations and the time of electrolysis. It was found that the hexavalent uranium was quantitatively reduced to trivalent state by mercury cathode electrolysis. The stability of uranium (Ⅲ) in various hydrochloric acid solution was measured in the air and nitrogen, and the kinetics of oxidation of uranium (Ⅲ) proceeded reasonably to be a first order reaction in the air. The absorption spectra of uranium (Ⅲ) in hydrochloric acid solution were examined.