The complex of Fe(NO3)3 and N,N,N′,N′-tetrakis(2-benzimidazolylmethyl)-2-hydroxy-1,3-diaminopropane (HPTB) is reformulated as [Fe2(HPTB)(μ-OH)(NO3)2](NO3)2 on the basis of 1H NMR, EXAFS, X-ray diffraction, and conductivity data. This complex reacts with hydrogen peroxide to form a 1:1 adduct with a new charge-transfer band at 600 nm (є = 1500 M-1 cm-1). Resonance Raman studies show two resonance-enhanced vibrations, vFe-o at 476 cm-1 and vo-o, which appears as a Fermi doublet centered at 895 cm-1; these features shift to 457 and 854 cm-1, respectively, with the use of H218O2 but are not affected by D2O. 1H NMR measurements indicate that the antiferromagnetic coupling is increased from -J = 20 cm-1 to ca. 70 cm-1 upon formation of the peroxide adduct, suggesting the introduction of a new coupling pathway. The 57Fe Mössbauer spectrum of the peroxide complex reveals a quadrupole doublet (δ - 0.54 mm/s, ΔEQ = 0.84 mm/s) distinct from that of the precursor complex (δ = 0.49 mm/s, ΔEQ = 0.66 mm/s), indicating that the two irons are affected similarly by peroxide binding. Conductivity measurements in CH3CN show that the adduct is a 1:1 electrolyte. Taken together, the physical data suggest the formulation [Fe2(HPTB)(μ-η1:η1-O2)-NO3)2](NO3) for the peroxide complex. Such dinuclear iron peroxide complexes may be relevant to putative intermediates in the oxygenation of the reduced forms of ribonucleotide reductase and methane monooxygenase.