Hypoiodous acid is an inorganic compound with the chemical formula HIO.

It forms when an aqueous solution of iodine is treated with mercuric or silver salts.

They rapidly disproportionate to form iodides and iodates,[2] but an iodine–hydroxide mixture can be used an in situ preparation of hypoiodite for other reactions.

[3] Hypoiodite is one of the active oxidizing agents generated by lactoperoxidase as part of the mammalian innate immune system.

[4][5] Hypoiodous acid is part of a series of oxyacids in which iodine can assume oxidation states of −1, +1, +3, +5, or +7.