Mechanistic studies of the reactions of three analogous alkoxo-bridged diiron(II) complexes with O-2 have been carried out. The compounds, which differ primarily in the steric accessibility of dioxygen to the diiron(lI) center, form metastable mu-peroxo intermediates when studied at low temperature. At ambient temperatures, these intermediates decay to form (mu-oxo)polyiron(III) products. The effect of ligand steric constraints on the O-2 reactivity was investigated. When access to the diiron center was unimpeded, the reaction was first-order with respect to both [Fe-2(II)] and [O-2] and the activation parameters for O-2 addition were similar to those for O-2 reacting with the dioxygen transport protein hemerythrin, When the binding site was occluded, however, reduced order with respect to [O-2] was observed and a two-step mechanism was required to explain the kinetic results. Decay of all three peroxide intermediates involves a bimolecular event, implying the formation of tetranuclear species in the transition state.