Phosphinous acid is the inorganic compound with the formula H<sub>2</sub>POH. It exists, fleetingly, as a mixture with its less stable tautomer H<sub>3</sub>PO (phosphine oxide). This mixture has been generated by low temperature oxidation of phosphine with ozone. H<sub>2</sub>POH is mainly of pedagogical interest. Organophosphinous acids are more prevalent than the parent H<sub>2</sub>POH.
Phosphinous acids exist mainly as minor tautomers of secondary phosphine oxides. For example diphenylphosphinous acid, which is not detectable directly, is invoked as the tautomer of diphenylphosphine oxide.
Highly electron-withdrawing substituents stabilize the phosphinous acid tautomer as illustrated by (CF<sub>3</sub>)<sub>2</sub>POH.