The coordination of the U(IV) and U(VI) ions as a function of the chloride concentration in aqueous solution has been studied by U LIII-edge extended X-ray absorption fine structure (EXAFS) spectroscopy. The oxidation state of uranium was changed in situ using a gastight spectroelectrochemical cell, specifically designed for the safe use with radioactive solutions. For U(VI) we observed the complexes UO2(H2O)5 2+, UO2(H2O)4Cl+, UO2(H2O)3Cl2 0, and UO2(H2O)2Cl3 - with Cl- increasing from 0 to 9 M, and for U(IV) we observed the complexes U(H2O)9 4+, U(H2O)8Cl3+, U(H2O)6 - 7Cl2 2+, and U(H2O)5Cl3 +. The distances in the U(VI) coordination sphere are U−Oax = 1.76±0.02 Å, Oeq = 2.41 ± 0.02 Å, and U−Cl = 2.71 ± 0.02 Å; the distances in the U(IV) coordination sphere are U−O = 2.41 ± 0.02 Å and U−Cl = 2.71 ± 0.02 Å.