The (mu-oxo)diiron(III) complexes of N,N'-bis(2-methyl-2-mercaptopropane)-1,5-diazacyclooctane (H(2)bme*-daco) and N-N'-bis(mercaptoethyl)-1,5-diazacyclooctane (H(2)bme*-daco) ligands present uncommon examples of well-characterized (mu-oxo)diiron(III) species in the presence of sulfur donors. Whereas the reaction of molecular oxygen with a solution of diiron(II) complex, [(bme*-daco)Fe](2), affords slow formation of the mu-oxo species, the presence of O-2 in the reaction of free Ligand with the iron source, Fe(acac)(3), gave good yields in shorter time. With the less sterically encumbered derivative, [(bme-daco)Fe](2), the only successful route to [(bme-daco)Fe](2)O required single O-atam sources such as 4-chloropyridine N-oxide to be present during the preparation of the complex; attempted use of molecular O-2 either during the synthesis or following isolation of the iron(II) dimer [(bme-daco)Fe](2) resulted in decomposition. That is, both the reactant [(bme-daco)Fe](2) and the product [(bme-daco)Fe](2)O are unstable toward Oz. The molecular structures of [(bme*-daco)Fe](2), [(bme*-daco)Fe](2)O, and [(bme- daco)Fe](2)O were determined by X-ray crystallography. The topological description of the Fe(II) dimers is that of edge-bridged square pyramids, with the Fe(II) displaced from the N2S2 planes of each ligand on average by 0.59 Angstrom and engaged in a planar Fe2S2 core. For the (mu-oxo)diiron(III) compounds, the square pyramids are O-vertex bridged, and the Fe(III) dome effect is 0.65 and 0.62 Angstrom for [(bme*-daco)Fe](2)O and [(bme-daco)Fe](2)O, respectively. Further characterization by ESI mass spectroscopy, cyclic voltammetry, and EPR spectroscopy (of the one-electron reduced [(bme*-daco)Fe](2)O is presented. There was no indication of the Ligand-based oxygen capture producing S-oxygenates which are prevalent in nickel derivatives of bme-daco and bme*-daco.