In the title phospho-nate, C19H21O9P, the dihedral angle between the benzene rings is 63.33 (3)°, and the P atom has a distorted tetra-hedral geometry, with angles in the range 101.30 (6)-120.38 (6)°. No significant inter-molecular inter-actions are observed in the crystal structure, and π-π inter-actions between symmetry-related benzene rings are beyond 4 Å.