Although they are derivatives of phosphonous acid (RP(OH)), they are not prepared from such precursors.  Phosphonites are prepared by alcoholysis of organophosphinous chlorides. For example, treatment of dichlorophenylphosphine with methanol and base gives dimethyl phenylphosphonite:
:ClPPh + 2 CHOH → (CHO)PPh + 2 HCl