Several [FeII2(N-EtHPTB)(μ-O2X)] 2+ complexes (1·O2X) have been synthesized, where N-EtHPTB is the anion of N,N,N′N′-tetrakis(2-benzimidazolylmethyl)- 2-hydroxy-1,3-diaminopropane and O2X is an oxyanion bridge. Crystal structures reveal five-coordinate (μ-alkoxo)diiron(II) cores. These diiron(II) complexes react with O2 at low temperatures in CH 2Cl2 (-90 C) to form blue-green O2 adducts that are best described as triply bridged (μ-η1:η1- peroxo)diiron(III) species (2·O2X). With one exception, all 2·O2X intermediates convert irreversibly to doubly bridged, blue (μ-η1:η1-peroxo)diiron(III) species (3·O2X). Where possible, 2·O2X and 3·O2X intermediates were characterized using resonance Raman spectroscopy, showing respective νO-O values of ∼850 and ∼900 cm-1. How the steric and electronic properties of O 2X affect conversion of 2·O2X to 3·O 2X was examined. Stopped-flow analysis reveals that oxygenation kinetics of 1·O2X is unaffected by the nature of O 2X, and for the first time, the benzoate analog of 2·O 2X (2·O2CPh) is observed.