Hypofluorous acid, chemical formula HOF, is the only known oxyacid of fluorine and the only known oxoacid in which the main atom gains electrons from oxygen to create a negative oxidation state.

HOF is explosive at room temperature, forming HF and O2:[1] This reaction is catalyzed by water.

[2] It was isolated in the pure form by passing F2 gas over ice at −40 °C, rapidly collecting the HOF gas away from the ice, and condensing it:[2] The compound has been characterized in the solid phase by X-ray crystallography[1] as a bent molecule with an angle of 101°.

The structure has also been analyzed in the gas phase, a state in which the H–O–F bond angle is slightly narrower (97.2°).

Where reduction of a general hypohalous acid reduces the halogen atom and yields the corresponding elemental halogen gas, reduction of hypofluorous acid instead reduces the oxygen atom and yields fluoride directly.