Tetraiodine nonoxide is an iodine oxide with the chemical formula I<sub>4</sub>O<sub>9</sub>.
Tetraiodine nonoxide can be produced by reacting ozone and iodine in carbon tetrachloride at âÂÂ78 ðC:
It can also be produced by heating iodic acid and phosphoric acid together:
Tetraiodine nonoxide is a light yellow solid that can easily hydrolyze. It decomposes above 75 ðC:
Like diiodine tetroxide, tetraiodine nonoxide contains both I(III) and I(V), and disproportionates to iodate and iodide under alkaline conditions:
It reacts with water to form iodic acid and iodine: