In nature, C-H bond oxidation of CH4 involves a peroxo intermediate that decays to the high-valent active species of either a "closed" {Fe-IV(mu-O)(2)Fe-IV} core or an "open" {Fe-IV(O)(mu-O)Fe-IV(O)} core. To mimic and to obtain more mechanistic insight in this reaction mode, we have investigated the reactivity of the bioinspired diiron complex [(susan){Fe(OH)(mu-O) Fe-(OH)}](2+) [susan = 4,7-dimethyl-1,1,10,10-tetrakis(2-pyridylmethyl)-1,4,7,10-tetraazadecane], which catalyzes CH3OH oxidation with H2O2 to HCHO and HCO2H. The kinetics is faster in the presence of a proton. O-18-labeling experiments show that the active species, generated by a decay of the initially formed peroxo intermediate [(susan){Fe-III(mu-O)(mu-O-2) Fe-III}](2+), contains one reactive oxygen atom from the mu-oxo and another from the mu-peroxo bridge of its peroxo precursor. Considering an (FeFeIV)-Fe-IV active species, a "closed" {Fe-IV(mu-O)(2)Fe-IV} core explains the observed labeling results, while a scrambling of the terminal and bridging oxo ligands is required to account for an "open" {Fe-IV(O)(mu-O)Fe-IV(O)} core.'