Cubic Pa3-type RuO 2 was prepared from the ambient pressure, rutile-structured phase at 20 GPa and 1100°C in a multianvil device. The structure of this Pa3 phase was refined by time-of-flight, neutron powder diffraction on the quenched sample yielding a cell constant a = 4.85892(3) and an oxygen positional parameter u = 0.35115(8). The ruthenium cation is rhombohedrally coordinated with six anions at 1.9893(4) and two more distant anions at 2.9552(4) . The minimum interpolyhedral O-O distance of 2.5045(5) in this structure is the shortest known in any solid and is shorter than the intrapolyhedral O-O distances, which are of 2.6208(5) . These short distances are the origin of the very low compressibility of this oxide phase, which approaches that of diamond. The Raman spectrum of Pa3-type RuO 2 is consistent with group-theoretical calculations.