The kinetics and mechanisms of the catecholase-type biomimetic activation of O-2 by the new dioximatoiron(II) complexes [Fe(Hdmed)](+), [Fe(Hdmpd)](+) and [Fe(H(2)dmdt)](2+) have been studied in methanol. Kinetic measurements reveal first order behavior with respect to catalyst and O-2 and a saturation type dependence on the 3,5-di-tert-butylcatechol (H(2)dtbc) substrate. Added triethylamine increases the rate by changing the reaction mechanism. An important, hitherto unknown feature is iron(II)-enhanced base catalysis of H(2)dtbc oxidation, via coordination of HdtbcO(2)(-) to the Fe(II) complex present, resulting in a significant acceleration of oxidation. A mechanism involving free radicals is suggested on grounds of ESR evidence. The activity pattern of the catalyst complexes correlates with coordination number and symmetry as revealed by Mossbauer spectra.