The structures and reactions of different rhodium oxides and dioxygen complexes with RhO₄ stoichiometry were investigated by matrix isolation infrared spectroscopy and quantum chemical calculations. The inserted RhO₂ molecule reacted with dioxygen upon sample annealing to form the [(η¹-O₂)RhO₂] complex, which can further isomerize to the known [(η²-O₂)RhO₂] complex via infrared irradiation. Both experimental and theoretical studies suggest that the [(η¹-O₂)RhO₂] complex has a doublet ground state with non-planar C s symmetry in which the O₂ ligand is end-on bonded to the rhodium centre. Although rhodium tetroxide is predicted to be a stable molecule with D ₂d symmetry at different level of theory, no evidence is found for the formation of this Rh(VIII) species in noble gas matrices. Our experiments also suggest the formation of a new peroxo [Rh(η²-O₂)₂] complex, which is calculated to have a doublet ground state with D ₂d symmetry. This peroxo complex undergoes isomerization to the known superoxo [Rh(η²-O₂)₂] complex via the rotation of the dioxygen ligand under infrared irradiation.