Iridium tetroxide (IrO<sub>4</sub>, Iridium(VIII) oxide) is a binary compound of oxygen and iridium in oxidation state +8. This compound was formed by photochemical rearrangement of (÷<sup>1</sup>-O<sub>2</sub>)IrO<sub>2</sub> in solid argon at a temperature of . At higher temperatures, the oxide is unstable. The detection of the iridium tetroxide cation by infrared photodissociation spectroscopy with formal oxidation state +9 has been reported, the highest currently known of any element. However no salts are known, as attempted production of an Ir(IX) salt such as IrO<sub>4</sub>SbF<sub>6</sub> did not result in anything.