In the title polymer, {[Cu(C(8)H(12)N(2)O(6)P(2))]·2H(2)O}(n), the geometry of the five-coordinate Cu(II) ion can best be described as slightly distorted square-pyramidal formed by one N and two O atoms of an N(CH(2)PO(3)H)(2) group and one N atom from a pyridine ring. The elongated apex of the pyramid is occupied by one O atom from a third diphospho-nate ligand. The inter-connection of Cu(2+) ions by the diphospho-nate ligands results in the formation of a double-chain array along the b axis, in which the two sub-chains are inter-locked by pairs of PO(3) groups. The outside of each sub-chain is decorated by other PO(3) groups. These double chains are further assembled into a three-dimensional supra-molecular architecture via a large number of O-H⋯O hydrogen bonds between the phospho-nate groups and lattice water mol-ecules.