Hypoiodous acid is an inorganic compound with the chemical formula HOI. It forms when an aqueous solution of iodine is treated with mercuric or silver salts. It rapidly decomposes by disproportionation: Hypoiodites of alkali and alkaline earth metals can be made in cold dilute solutions if iodine is added to their respective hydroxides. Hypoiodous acid is a weak acid with a pKa of about 11. The conjugate base is hypoiodite (IO−). Salts of this anion can be prepared by treating I2 with alkali hydroxides. They rapidly disproportionate to form iodides and iodates.

Other oxyacids

Hypoiodous acid is part of a series of oxyacids in which iodine can assume oxidation states of −1, +1, +3, +5, or +7. A number of neutral iodine oxides are also known.