The title compound, C(7)H(9)O(4)P, obtained as a by-product of the reaction between Lawesson's reagent, (I), and CH(3)I, can be recognized as the final product of the S/O interchange reaction at the P atom of (I). Hydrogen bonds of type P-O-H.O=P link molecules into helical chains and form ten-membered hydrogen-bonded rings with the graph-set notation R(3)(2)(10). Weaker intermolecular contacts between P-O and a phenyl H atom link the chains into a three-dimensional lattice. The parent benzenephosphonic acid [Weakley (1976). Acta Cryst. B32, 2889-2890] does not adopt an analogous structure, but its arsenic analogue [Shimada (1960). Bull. Chem. Soc. Jpn, 33, 301-304] does and can be regarded as isostructural. We rationalize these three structures in terms of their significant intermolecular interactions.