Equilibria in aqueous solutions at ionic strength I = 1.00 (NaClO4 or NaNO3) and 25 degrees C between the 1:1 thallium(III) complexes [T1(L)] (L are diaminotetraacetate ligands edta and cdta and the hexaamine tpen {tetrakis(2-pyridylmethyl)ethylenediamine}) and different coligands A (A are OH-, Cl-, Br-, I-, SCN-, N-3(-), py, en, Hen(+), phen, bpy, gly(-), Hgly, ox(2-), and ida(2-)) have been investigated by means of potentiometric pH titrations and C-13 and H-1 NMR, Raman, and IR spectroscopy. Each ligand gives rise to the following equilibria, allowing the determination of the corresponding equilibrium constants by pH measurements : (1) [Tl(L)(H2O)](n) reversible arrow[Tl(L)(OH)](n-1) + H+; (2) [Tl(L)(H2O)](n) + A reversible arrow [Tl(L)(A)](n) + H2O.The presence of [Tl(L)(A)] in solution and the corresponding data have been confirmed by NMR for [Tl(edta)(A)] and [Tl(cdta)(A)]: Although the C-13 coordination chemical shifts are small, heteronuclear coupling constants J(Tl-205/203, C-13) and, in particular, those involving the carboxylate function can be employed for monitoring structural changes and the determination of equilibrium constants. The cdta complexes, having a rigid stereochemistry imposed by the cyclohexyl ring, in solution exhibit conformations for the glycine-type chelate rings which are close to those found in the solid for analoguous edta complexes. The parameter (3)J(T1,H) for the acetate groups of the cdta ligand established conformational similarities for all the complexes with monodentate coligands A, different from those coordinating in a bidentate fashion, suggesting that also the complex [Tl(L)(H2O)(n)](-) exists as a monoaquo species, i.e., n = 1. Two-dimensional NMR for [Tl(cdta)(phenH)] and [Tl(cdta)(phen)](-) provides simple and direct evidence for the formation of stable mixed-ligand complexes with CN = 7 and 8 in aqueous solution. Raman spectra of solutions with equilibria of types (1) and (2) for L = edta allow the assignment of bands to the mixed-ligand complexes present in agreement with the equilibrium constants obtained. The X-ray crystal structures of two complexes which were isolated in crystalline form, i.e., Ca[Tl(edta)(OH)]. 3H(2)O (monoclinic, space group P2(1)/c, a = 13.744(8) Angstrom, b = 9.141(2) Angstrom, c = 13.752(0) Angstrom, beta = 100.71(0)degrees, Z = 4) and [Tl(tpen)(NO3)](ClO4)(2) (monoclinic, space group P2(1)/c, a = 12.718(6) Angstrom, b = 13.640(7) Angstrom, c = 18.883(9) Angstrom, beta = 108.84(4)degrees, Z = 4) showed a CNs of 7 for the former and 8 for the latter Tl(III) complex salt.