RuO 4 was prepared by oxidation of elemental ruthenium. Two different modifications were obtained and investigated by X-ray single crystal diffraction. RuO 4-I has cubic symmetry (P4;-3n,Z=8,a=8.509(1) Å), and two independent tetrahedral molecules are present in the unit cell. Within the standard uncertainties in both molecules the distances Ru-O are 1.695 Å. The second modification, RuO 4-II, is monoclinic (C2/c,Z=4,a=9.302(4) Å,b=4.3967(10) Å,c=8.454(4) Å,β=116.82(3)°) and isotypic with OsO 4. There is one independent molecule in the unit cell, which shows distances Ru-O of 1.697 and 1.701 Å, respectively.