The mixed-valence compound bis(mu-oxo)tetrakis(2,2’-bipyridine)dimanganese(III,IV) is a small molecule model compound for the oxygen-evolving center, an enzyme which converts water into dioxygen in photosystem II in the photosynthesis of green plants and algae. Crystallographic data : M(r) 1077.47 g/mol, monoclinic, P2(1)/c, Z = 4, a = 13.584(2) Angstrom, b = 14.058(4) Angstrom, c = 23.622(5) Angstrom, beta = 105.25(2)degrees, V = 4352(4) Angstrom(3) at T = 9 K. A quantitative estimate for the valence contrast, i.e., the difference in electrostatic potential of the two Mn atoms in this compound, has been established by two different experimental methods which both involve single-crystal X-ray diffraction. A resonant synchrotron X-ray diffraction experiment examining the Bragg scattering for energies of the incoming beam close to the Mn absorption edge at 6539 keV (similar to 1.9 Angstrom wavelength) shows a 3.7 eV difference in ionization energies of the first K-shell electron for Mn(IV) and Mn(III), respectively. : M(r) 1077.47 g/mol, monoclinic, P2(1)/c, Z = 4, a = 13.584(2) Angstrom, b = 14.058(4) Angstrom, c = 23.622(5) Angstrom, beta = 105.25(2)degrees, V = 4352(4) Angstrom(3) at T = 9 K. A quantitative estimate for the valence contrast, i.e., the difference in electrostatic potential of the two Mn atoms in this compound, has been established by two different experimental methods which both involve single-crystal X-ray diffraction. A resonant synchrotron X-ray diffraction experiment examining the Bragg scattering for energies of the incoming beam close to the Mn absorption edge at 6539 keV (similar to 1.9 Angstrom wavelength) shows a 3.7 eV difference in ionization energies of the first K-shell electron for Mn(IV) and Mn(III), respectively.