The formation of complexes species of the dioxouranium(VI) ion with aspartic and glutamic acids was studied in the pH range of 3 to 6 at 25 degrees C by potentiometric measurements (H(+)-glass electrode). Results gave evidence for the formation of the following species: (UO(2))A(0), (UO(2))AH(+), and (UO(2))(2)A(OH)(2)(0) (A(2-) = a glutamic or aspartic ligand). Investigations were carried out in a NaCl ionic medium at I (0.1, 0.25, 0.5, and 1.0) mol L(-1). The dependence on ionic strength of the formation constants was analyzed by the specific ion interaction theory (SIT) model. The formation constants at infinite dilution, obtained using this model, are log beta(110) = 8.53 +/- 0.03, 8.37 +/- 0.05; log beta(111) = 13.60 +/- 0.05, 13.42 +/- 0.02; and beta(21-2) = 3.31 +/- 0.05, 2.98 +/- 0.03 for glutamate and aspartate, respectively, where indexes for the overall formation constant beta(pqr) refer to the equilibrium pUO(2)(2+) + qA(2-) + rH(+) = (UO(2))(p)(A)(q)H(r),((2p) (-) (2q + r)). The specific interaction coefficients are also reported. A mononuclear hydroxo species (UO(2))A(OH)(-), of great interest for very low naturally occurring concentrations of uranium(VI), was hypothesized, with equilibrium constant log K [(UO(2))A(0) + H(2)O = (UO(2))A(OH)(-)] = -5.4 at t = 25 degrees C and I = 0 mol L(-1). Speciation profiles for different concentrations Of UO(2)(2+) in the presence of both amino acids are discussed.