Diiodine oxide, also known as iodo hypoiodite, is an oxide of iodine that is equivalent to an acid anhydride of hypoiodous acid.

This substance is unstable and it is very difficult to isolate.

[2] Diiodine oxide can be prepared by reacting iodine with potassium iodate (KIO3) in 96% sulfuric acid and then extracting it into chlorinated solvents.

[2] Diiodine oxide reacts with water to form hypoiodous acid: