Diorganophosphites are derivatives of phosphorus(V) and can be viewed as the di-esters of phosphorous acid ((HO)P(O)H). They exhibit tautomerism, however, the equilibrium overwhelmingly favours the right-hand (phosphonate-like) form:
:(RO)POH ⇌ (RO)P(O)H
The P-H bond is the site of high reactivity in these compounds (for example in the Atherton–Todd reaction and Hirao coupling), whereas in tri-organophosphites the lone pair on phosphorus is the site of high reactivity. Diorganophosphites do however undergo transesterification.