Electrochemical behavior and interconversion of mono, bi and tetranuclear complexes have been investigated in aqueous phen+phenH(+) buffer (phen = 1, 10-phenanthroline). A controlled potential oxidation of [(Mn2O2)-O-III,IV(phen)(4)](3+) allows the quantitative formation of the new high valent oxo complexes [(Mn4O6)-O-IV(phen)(6)](4+). Associated electrochemical back transformations have also been studied.