Aqueous solution studies regarding the identification and characterization of complexes formed by the (VO)-O-IV ion and 11 3-hydroxy-4-pyridinone derivatives have been performed using EPR and UV/vis spectroscopic techniques. For the three ligands (HL) adequately soluble in water (1-methyl-3-hydroxy-4-pyridinone, 1-methyl-2-ethyl-3-hydroxy4- pyridinone, and 1,2-diethyl-3-hydroxy-4-pyridinone), potentiometric titrations were performed; the results are consistent with the formation of [(VOL)-O-IV](+), [(VOL2)-O-IV], [(VOL2H-1)-O-IV](-), [((VO)-O-IV)(2)L2H-2], and [(VL3)-L-IV](+) species. Bis chelated complexes are characterized by a cis - trans isomerism, the trans isomer being strongly favored with respect to the cis arrangement. Tris chelated non-oxo V-IV species were prepared in CH3COOH; their spectroscopic features point to a d(z)(2) ground state and a geometry intermediate between an octahedron and a trigonal prism, related to the steric requirements of the substituent on the carbon atom in position 2 of the pyridinone ring. Four new solid derivatives, [(VO)-O-IV(1,2-diethyl-3-hydroxy-4-pyridinonato)(2)], [(VO)-O-IV(1-(p-tolyl)-2-ethyl-3-hydroxy-4-pyridinonato)(2)], [(VO)-O-IV(1-(p-(n-butyl) phenyl)-2-ethyl-3-hydroxy-4-pyridinonato)(2)], and [(VO)-O-IV(1-(p-(n-hexyl)phenyl)-2-ethyl-3-hydroxy-4-pyridinonato) (2)], were isolated and characterized; they exhibited a five-coordinate geometry close to square- pyramid. A criterion for establishing the degree of distortion toward the trigonal-bipyramid on the basis of the electronic absorption spectra is provided. Relationships between the p K a of the - OH group in position 3 of the ring and (i) log K of mono and bis chelated complexes, (ii) pK of the water molecule in cis[(VOL2)-O-IV(H2O)], (iii) log K of tris chelated species [(VL3)-L-IV](+), and (iv) V-51 hyperfine coupling constant (A(z)) have been established and discussed for a number of pyrone, pyridinone, and catechol ligands. The results are rationalized by assuming for pyridinones an electronic structure intermediate between that of pyrones and catechols. The relationships are valuable to the understanding of the behavior of (VO)-O-IV species in aqueous solution.