The clusterfullerene Sc4O2@C80 with a mixed redox state of scandium was found to be an exciting molecule for endohedral electrochemistry as demonstrated by means of an in situ electron spin resonance (ESR) spectroelectrochemical study of the spin density distribution in its electrochemically generated cation and anion radicals. The compound exhibits two reversible reduction and oxidation steps with a relatively small electrochemical gap of 1.10 V. The ESR spectra of the ion radicals have a rich hyperfine structure caused by two pairs of equivalent Sc atoms. The Sc-based hyperfine structure with large hyperfine coupling constants shows that both oxidation and reduction of Sc4O2@C80 are in cavea redox processes, which is the subject of endohedral electrochemistry. The assignment of the experimentally determined a(45Sc) values to the two types of Sc atoms in the Sc4O2 cluster was accomplished by extended density functional theory and molecular dynamics simulations. Sc atoms adopting a divalent state in the neutral Sc4O2@C80 exhibited an especially large coupling constant of 150.4 G in the cation radical, which is the record high a(45Sc) value for Sc-based endohedral metallofullerenes. Such a high value is explained by the nature of the highest occupied molecular orbital (HOMO) localized on the six-atom Sc4O2 cluster. This HOMO is a Sc?Sc bonding MO and hence has large contributions from the 4s atomic orbitals of ScII. We claim that ESR spectroelectrochemistry is an invaluable experimental tool in the studies of metal?metal bonding in endohedral metallofullerenes and in endohedral electrochemistry.