EDTA forms stable complexes with plutonium that are integral to nuclear material processing, radionuclide decontamination, and the potentially enhanced transport of environmental contamination. To characterize the aqueous Pu(4+/3+)EDTA species formed under the wide range of conditions of these processes, potentiometry, spectrophotometry, and cyclic voltammetry were used to measure solution equilibria. The results reveal new EDTA and mixed-ligand complexes and provide more accurate stability constants for previously identified species. In acidic solution (pH < 4) and at 1:1 ligand to metal ratio, PuY (where Y4- is the tetra-anion of EDTA) is the predominant species, with an overall formation constant of log beta(110) = 26.44. At higher pH, the hydrolysis species, PuY(OH)(-) and PuY(OH)(2)(2-), form with the corresponding overall stability constants log beta(11-1) = 21.95 and 109 beta(11-2) = 15.29, The redox potential of the complex PuY at pH = 2.3 was determined to be E-1/2 = 342 mV. The correlation between redox potential, pH, and the protonation state of PuY- was derived to estimate the redox potential of the pU(4+/3+)y complex as a function of pH. Under conditions of neutral pH and excess EDTA relative to Pu4+, PuY24- forms with an overall formation constant of log beta(120) = 35.39. In the presence of ancillary ligands, mixed-ligand complexes form, as exemplified by the citrate and carbonate complexes PuY(citrate)(3-) (log beta(1101) = 33.45) and PuY(carbonate)(2)(log beta(1101) = 35.51). Cyclic voltarnmetry shows irreversible electrochemical behavior for these coordinatively saturated Pu4+ complexes: The reduction wave is shifted approximately -400 mV from the reduction wave of the complex PuY, while the oxidation wave is invariant.