The title compound, C(12)H(19)O(4)P, has a distorted tetra-hedral geometry around the P atom. The molecules form dimers with R(2) (2)(10) ring motifs due to inter-molecular O-H⋯O hydrogen bonds. The double-bonded O atom of the phospho-nate group behaves as an acceptor and the hydr-oxy group acts as a donor. Both of the ethyl groups are disordered with occupancies of 0.55:0.45 and 0.725:0.275.