Selenous acid, H₂SeO₃, is an oxoacid of selenium.

Selenous acid is analogous to sulfurous acid, but it is quite definitely existent and stable, found as selenous acid molecules in solution, and obtainable as a crystalline solid.

Selenous acid is easily formed by adding selenium dioxide to water.

Selenous acid has a layer structure in the solid state, with SeO₃ pyramids linked by hydrogen bonds. It is moderately oxidizing in nature, but kinetically slow.

In 1 M H⁺ :H₂SeO₃ + 4H⁺ + 4e⁻ Se + 3H₂O E^o = +0.74 V

In 1 M OH⁻ : SeO₃²⁻ + 4e⁻ + 3H₂O Se + 6OH⁻ E^o = −0.37 V

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Selenites | Acids