Intermediates of the catalytic oxidation of 3,5-di-tert-butylcatechol (3,5-DTBCatH(2)) by O-2 in the presence of (triphenylphosphine)bis(dimethylglyoximato)cobalt(II) as catalyst have been studied. The complex [Co(Hdmg)(2)-(Ph(3)P)(3,5-DTBCatH)] has been isolated from the reaction mixture and characterized by X-ray diffraction (monoclinic, P2(1)/c, a = 16.498(1) Angstrom, b = 9.254(1) Angstrom, c = 26.038(2) Angstrom, beta = 103.59(1)degrees, V = 3864.0(4) Angstrom(3), D-c = 1.33 g cm(-3), Z = 4, mu(Cu K alpha) = 44.4 cm(-1), R = 0.0480). It contains a unidentate 3,5-di-tert-butylcatecholato-(1-) ligand. Upon dissolution and warming in benzene under N-2, homolytic Co-O bond cleavage produces cobaloxime(II) and the free 3,5-di-tert-butylsemiquinonato(1-) anion radical, as demonstrated by ESR spectroscopy. In O-2-containing solution, the complex restarts the catalytic reaction, during which the [Co-III(Hdmg)2(Ph(3)P>(3,5-DTBSQ(.))] species can also be detected by ESR. The observed behavior is rationalized by a mechanism incorporating the intermediates identified.