The coordination circumstances and redox reactions of UO 2 2? in the aqueous solution concentrated by calcium chloride, such as CaCl 2 Á6H 2 O (6.9 M CaCl 2 ), were studied by Raman spectroscopy and electrochemical methods. The frequency of the O=U=O symmetrical stretching vibration suggested that the complex formation of UO 2 2? with Cl -leads to the weakening of U=O bond. In the electrochemical measurements, two-step cathodic currents were observed at -0.090 and -0.4 V (vs. Ag|AgCl) corresponding to the reduction of UO 2 2? to UO 2 ? and that of UO 2 ? to UO 2 , respectively. It was found that UO 2 ? formed at first cathodic current was disproportionated to form UO 2 2? and UO 2 . The UO 2 was identified by X-ray diffraction analysis. Electrolytic deposition of UO 2 was observed in 6.9-4.7 M CaCl 2 and in 14 M LiCl. When small amount of proton, i.e., 0.005 M was coexisted in 6.9 M CaCl 2 , UO 2 2? was reduced to form U 4? instead of UO 2 .