An excellent fit to a symmetric adamantanoid "Ru4O64+" core arrangement 1 (Ru-O mu, 1.83 Angstrom; Ru-OH2, 2.16 Angstrom; and Ru-Ru, 3.4 Angstrom) has been obtained from a ruthenium K edge EXAFS study on the tetrameric aqua ruthenium(IV) cation in both aqueous 2 M perchloric and nitric acid solution. Mononuclear "ruthenyl" and tetranuclear hydroxo-bridged formulations such as Ru-4(OH)(12)(4+)(aq) can now be unequivocally ruled out as relevant to the core structure.[GRAPHICS]