Chromium(V) oxalato complexes have been of considerable interest as intermediates in the Cr(VI) oxidation of oxalic acid, which is used as a prototype in mechanistic studies of Cr(VI) oxidations of organic substrates. These complexes have been characterized by X-band EPR spectroscopy from the reactions of Cr(VI) (CrO3 or Cr(V) (Na[(CrO)-O-V(ehba)(2)]; ehba = 2-ethyl-2-hydroxybutanoate(2-)) with oxalic acid (oxH(2)) in acidic aqueous solutions (pH = 0-1.5; I = 1 M (HClO4/NaClO4); 21 degrees C). Structures and formation mechanisms of these complexes have been deduced from the dependences of the relative intensities of the corresponding EPR signals (obtained by digital simulations of the EPR spectral on the reaction conditions. A range of Cr(V) complexes, including five-coordinate species, [Cr-V(O)(2)(OH2)(ehba)](-), [(CrO)-O-V(ehba)(ox)](-), and [(CrO)-O-V(ox)(2)](-), and six-coordinate species, [(CrO)-O-V(OH2)(OX)(2)](-), [(CrO)-O-V(OH)(ox)(2)](2-), [(CrO)-O-V(oxH)(OX)(2)](2-), and possibly [Cr-V(O)(2)(OH2)(2)(ox)](-), as well as mixed-valence Cr(III)-Cr(V) dimers and trimers and Cr(VI)-Cr(V) species, such as [(CrCrVI)-Cr-V(O)(5)(OH2)(ox)(2)](3-), have been characterized. Apart from the first detailed characterization of these species, this is the first spectral evidence for the existence of mixed-valence Cr(V)-Cr(VI) complexes. On the basis of quantitative analysis of EPR spectra for the Cr(VI)-oxH(2) system, Cr(VI) has been shown to exist mainly in the form of the monooxalato complex, [Cr-VI(O)(2)(OH)(ox)](-) in the presence of excess OXH2. The data on the structures and reactivities of Cr(V) and Cr(VI) oxalato complexes have been used to propose a new mechanism for the Cr(VI) oxidation of oxH(2) in acidic aqueous solutions.