Hafnium atom oxidation by dioxygen molecules has been investigated using matrix isolation infrared absorption spectroscopy. The ground-state hafnium atom inserts into dioxygen to form primarily the previously characterized HfO2 molecule in solid argon. Annealing allows the dioxygen molecules to diffuse and react with HfO2 to form OHf(eta(2)-O-2)(eta(2)-O-3), which is characterized as a side-on bonded oxo-superoxo hafnium ozonide complex. Under visible light (532 nm) irradiation, the OHf(eta(2)-O-2)(eta(2)-O-3) complex either photochemically rearranges to a more stable Hf(eta(2)-O-2)(3) isomer, a side-on bonded di-superoxo hafnium peroxide complex, or reacts with dioxygen to form an unprecedented homoleptic tetra-superoxo hafnium complex: Hf(eta(2)-O-2)(4). The Hf(eta(2)-O-2)(4) complex is determined to possess a D-2d geometry with a tetrahedral arrangement of four side-on bonded O-2 ligands around the hafnium atom, which thus presents an 8-fold coordination. These oxygen-rich complexes are photoreversible; that is, formation of Hf(eta(2)-O-2)(3) and Hf(eta(2)-O-2)(4) is accompanied by demise of OHf(eta(2)-O-2)(eta(2)-O-3) under visible (532 nm) light irradiation and vice versa with UV (266 nm) light irradiation.