The crystal structure of the title compound, C(12)H(12)O(6)P(2), displays two different regions alternating along the a axis: a hydrogen-bonded region encompassing the end-positioned phosphonic acid groups and a hydrophobic region formed by the aromatic spacers. The asymmetric unit contains only half of the biphenyl-4,4'-diphosphonic acid (4,4'-bpdp) molecule, which is symmetric with an inversion centre imposed at the mid-point between the two aromatic rings. The periodic organization of the molecules is controlled by two strong O-H...O interactions between the phosphonic acid sites. Weak C-H...pi interactions are established in the aromatic regions.