X-ray crystal structure of di[aqua(1-oxyethylidenediphosphonato(1-))]copper(II) dihydrate Cu[CH3C(OH)(P2O6H3) · H2O]2 · 2H2O is determined. These crystals are monoclinic (space group C2/c) at 24°C, a = 15.021(6) Å, b = 11,364(5) Å, c = 12.583(7) Å, β = 118.67(4)○, Z = 4, and V = 1885(2) Å3. Copper atoms form a complex whose geometry is a distorted octahedron with two bidentate-chelating ligands [CH3C(OH)(P2O6H3)]- in the equatorial plane (Cu-O bond lengths are 1.953(2) and 1.976(2) Å) and two apical water molecules (Cu-O bond lengths are 2.469 Å). The ligand in the equatorial plane forms a six-member chelate ring having a sofa conformation, in which one carbon atom deviates by 0.821 Å from the plane of other coplanar atoms. Because of the strong hydrogen bond P-O⋯H-O-P (2.506 Å long) between the neighboring coordination groups, the P-O double bond is elongated to 1.511 (2) Å and the ordinary bond P-OH is shortened to 1.537(2) Å.