The coordination circumstances and redox reactions of UO2 (2+) in the aqueous solution concentrated by calcium chloride, such as CaCl2 center dot 6H(2)O (6.9 M CaCl2), were studied by Raman spectroscopy and electrochemical methods. The frequency of the O=U=O symmetrical stretching vibration suggested that the complex formation of UO2 (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 UO2 (2+) to UO2 (+) and that of UO2 (+) to UO2, respectively. It was found that UO2 (+) formed at first cathodic current was disproportionated to form UO2 (2+) and UO2. The UO2 was identified by X-ray diffraction analysis. Electrolytic deposition of UO2 was observed in 6.9-4.7 M CaCl2 and in 14 M LiCl. When small amount of proton, i.e., 0.005 M was coexisted in 6.9 M CaCl2, UO2 (2+) was reduced to form U4+ instead of UO2.