With the goal of gaining insight into the structures of peroxo intermediates observed for oxygen-activating nonheme diiron enzymes, a series of metastable synthetic di iron (III)-peroxo complexes with [Fe-2(III)(mu-O)(mu-1,2-O-2)] cores has been characterized by X-ray absorption and resonance Raman spectroscopies, EXAFS analysis shows that this basic core structure gives rise to an Fe-Fe distance of similar to 3.15 angstrom; the distance is decreased by 0.1 angstrom upon introduction of an additional carboxylate bridge. In corresponding resonance Raman studies, vibrations arising from both the Fe-O-Fe and the Fe-O-O-Fe units can be observed, Importantly a linear correlation can be discerned between the v(O-O) frequency of a complex and its Fe-Fe distance among the subset of complexes with [Fe-2(III)(mu-OR)(mu-1,2-O-2)] cores (R = H, alkyl, aryl, or no substituent). These experimental studies are complemented by a normal coordinate analysis and DFT calculations.