Rb4CO4 crystallizes in the cubic P-43m space group. The structure is three-dimensional. Rb1+ is bonded in a 6-coordinate geometry to six equivalent O2- atoms. There are three shorter (2.67 Å) and three longer (3.50 Å) Rb–O bond lengths. C4+ is bonded in a tetrahedral geometry to four equivalent O2- atoms. All C–O bond lengths are 1.45 Å. O2- is bonded in a single-bond geometry to six equivalent Rb1+ and one C4+ atom.