The 4H(+)/4e(-) reduction of O-2 to water, a key fuel cell reaction also carried out in biology by oxidase enzymes, includes the critical O-O bond reductive cleavage step. Mechanistic investigations on active-site model compounds, which are synthesized by rational design to incorporate systematic variations, can focus on and resolve answers to fundamental questions, including protonation and/or H-bonding aspects, which accompany electron transfer. Here, we describe the nature and comparative reactivity of two low-spin heme peroxo Cu complexes, LS-4DCHIm, [(DCHIm)F8FeIII-(O-2(2-))Cu-II(DCHIm)(4)](r), and LS-3D CHIm, CHIm)F8FeIII-(O-2(2-))-Cu-II(DCHIm)(3)](+) (F-8 = tetrakis(2,6-difluorophenyl)-porphyrinate; DCHIm = 1,5-dicyclohexylimidazole), toward different proton (4nitrophenol and [DMF center dot H+](CF3SO3)) (DMF = dimethylformamide) or electron (decamethylferrocene (Fc*)) sources. Spectroscopic reactivity studies show that differences in structure and electronic properties of LS-3DCHIm and LS-4DCHIm lead to significant differences in behavior. LS-3DCHIm is resistant to reduction, is unreactive toward weakly acidic 4-NO2 phenol, and stronger acids cleave the metal-0 bonds, releasing H2O2. By contrast, LS-4DCHIm forms an adduct with 4-NO2 phenol, which includes an H-bond to the peroxo 0-atom distal to Fe (resonance Raman (rR) spectroscopy and DFT). With addition of Fc* (2 equiv overall required), O-O reductive cleavage occurs, giving water, Fe(III), and Cu(II) products; however, a kinetic study reveals a one-electron rate-determining process, ket = 1.6 M-1 s(-1) (-90 degrees C). The intermediacy of a high-valent [(DCHIm)F8FeIV=-0] species is thus implied, and separate experiments show that one-electron reduction-protonation of [(DCHIm)F8FeIV=0] occurs faster (k(et2) = 5.0 M-1 s(-1)), consistent with the overall postulated mechanism. The importance of the H-bonding interaction as a prerequisite for reductive cleavage is highlighted.