The coordination chemistry of dioxouranium(VI) with organic ligands may represent an useful model to describe the migration of radionuclides in natural fluids. A new dioxouranium(VI) ternary complex of formula Na-4(UO2)(2)(OH)(4)(C2O4)(2) was precipitated at pH congruent to 8. It was characterized by elemental analysis, ICP and IR spectroscopy, thermogravimetry (TG) and coupled TG-IR evolved gas analysis. Potentiometric measurements in the solution suggest that the equilibrium shifts from the prevalence of the (UO2)(2)(OH)(2)(C2O4)(4)(6-) complex to the [(UO2)(2)(OH)(4)(C2O4)(2)](4-) complex before the formation of a solid phase.