Stopped-flow kinetic studies of the reactions of [Fe-2(BIPhMe)(2)(O2CH)(4)] (1a), where BIPhMe = 2,2’-bis(1-methylimidazolyl)phenylmethoxymethane, and [Fe-2(OH)(Me(3)TACN)(2)(OAc)(2)](+) (2a), where TACN = 1,4,7-trimethyl-1,4,7-triazacyclononane, with O-2 are presented. In CHCl3 both reactions are first-order in [O-2] and secondorder in diiron(II) complex concentration. Mechanisms consistent with the kinetic data are proposed. The main feature of these schemes is a bimolecular pathway involving a tetranuclear (mu(4)-peroxo)diiron(II)diiron(III) species in the transition state. Evidence for carboxylate shifts in the oxidation mechanisms is presented, offering one possible way in which this class of structural equilibria might control the chemistry at related non-heme diiron centers in metalloproteins.