The vanadium(V) and -(IV) complexes formed with N-(phosphonomethyl)iminodiacetate (pida(4-)) complexes in water and in aqueous mixed-solvent systems were characterized by employing potentiometry, multinuclear NMR, EPR, and visible spectroscopy. The vanadium(V) complexes in water had a 1:1 metal to ligand stoichiometry. The diprotonated vanadium(V)-pida complex with a charge of - I existed below pH 2. After the first deprotonation, the dianionic species was stable from pH 1 to 6. The trianionic species was stable at neutral pH. Observations are most consistent with the protonation sites of these complexes on the organic ligand. Intra- and intermolecular exchange professes were identified using C-13 homonuclear EXSY spectroscopy. Given the asymmetric vanadium atom in the VO(2)pida(3-) complex, one of the exchange processes observed was the conversion of one enantiomer to the other in an equilibrium mixture. Studies of the corresponding vanadium(IV)-pida system in dioxane - water mixtures showed the presence of not only mononuclear species but also a dinuclear species (which was recently characterized by X-ray crystallography). In 80:20 (v/v) dioxane-water mixtures, the dinuclear species became the major species in solution, as evidenced by absorption spectroscopy and EPR spectroscopy. Potentiometric measurements in mixed solvents failed to conclusively favor any stoichiometry. In conclusion, the mononuclear vanadium(IV) and -(V) complexes with the H(4)pida ligand appear to be structurally related and could readily be distinguished from the dinuclear vanadium(IV)-pida complex that was characterized both in the solid state and in mixed-solvent solutions.