Iodine dioxide is a binary inorganic compound of iodine and oxygen with the chemical formula IO2.

[1][2] Only stable as a dilute gas,[3] this compound is one of many iodine oxides,[4][5] and "iodine dioxide" is sometimes used to describe its formal dimer, the salt diiodine tetroxide (I2O4, [IO]+[IO3]−).

It is formed, in parts-per-trillion quantities,[6] at the marine boundary layer and believed to mediate particulate nucleation therein.

The iodine, originally from algaeic diiodomethane, is photooxidized to iodine monoxide, which then disproportionates to free iodine atoms and iodine dioxide.

At a sufficiently large concentration, the particles then combine to form small grains of diiodine tetroxide.