text: According to the analysis by electron paramagnetic resonance spectroscopy, the resting state of the FeMo cofactor has a spin state of S=3/2. Upon one-electron reduction, the cofactor becomes EPR silent. Understanding the process in which an electron is transferred in the protein adduct shows a more precise kinetic model of the FeMo cofactor. Density functional theory calculations as well as spatially resolved anomalous dispersion refinement have suggested that the formal oxidation state is Mo-2Fe-5Fe-C-H, but the "true" oxidation states have not been confirmed experimentally.