Orthorhombic (O) RbClO4 transforms to a new tetragonal (T) form at pressures above 20 kbar. Thermal decomposition proceeds through the reversible equilibrium reaction T RbClO4 = B2 RbCl + 2O2 at modestly increasing temperatures centered ~550¿C over the pressure range investigated from 20 to 90 kbar. Cell volumes for T RbClO4 and B2 RbCl were determined by high-pressure, high-temperature, in situ X-ray diffraction on station 16.4 of the Daresbury Synchrotron Radiation Source, from which the partial molar volume of O2 along the decomposition curve was determined. This volume (~10 cm3/mol) is in close agreement with volumes determined from studies of the thermal decomposition of KClO3 and KClO4 but is difficult to reconcile with fluid O2 volumes determined using shock wave techniques. If these new O2 partial molar volumes are relevant to molten iron solutions, then storage of substantial amounts of O2 in the core becomes feasible.