Phosphinous acid is the inorganic compound with the formula H2POH.

It exists, fleetingly, as a mixture with its less stable tautomer H3PO (phosphine oxide).

This mixture has been generated by low temperature oxidation of phosphine with ozone.

Organophosphinous acids are more prevalent than the parent H2POH.

Highly electron-withdrawing substituents stabilize the phosphinous acid tautomer as illustrated by (CF3)2POH.