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The complex formation of the uranyl ion, UO22+, with chloride ions in acetonitrile has been investigated by factor analysis of UV-vis absorption and U L3 edge EXAFS (extended X-ray absorption fine structure) spectra. As a function of increasing [Cl-]/[UO22+] ratio, the five monomeric species [UO2(H2O)5]2+, [UO2Cl(H2O)2(MeCN)2]+, [UO2Cl2(H2O)(MeCN)2], [UO2Cl3(MeCN)2]-, and [UO2Cl4]2- have been observed. The distances determined in the first coordination sphere are: U-Oax = 1.77 Å, U-OH2O = 2.43 Å, U-NMeCN = 2.53 Å, and U-Cl = 2.68 Å. A crystalline material has been obtained from the intermediate solution with the [Cl-]/[UO22+] ratio of ~2, where [UO2Cl2(H2O)(MeCN)2] is the dominating species. The crystal structure analysis of this material revealed a tetrameric complex, [(UO2)4(μ2-Cl)4(μ3-O)2(H2O)2(CH3CN)4] · (CH3CN). The crystal data are: monoclinic, space group P21/n, a = 10.6388(5) Å, b = 14.8441(5) Å, c = 10.8521(5) Å, β = 109.164(5)°, and Z = 2. The U(VI) coordination of the solution species [UO2Cl2(H2O)(MeCN)2] changes during the crystallization by replacing one MeCN molecule with a bridging μ3-O atom in the tetramer. [ABSTRACT FROM AUTHOR]