The structure of the title polymer, }[Mg(C6H14N2O6P2)(H2O)4]·0.5H2O} n , is based on centrosymmetric MgO6 octahedra, which are linked by [(piperazine-1,4-diium-1,4-di-yl)bis-(methyl-ene)]di-phospho-nate ligands, forming chains parallel to [1-1-1]. These chains are connected via hydrogen bonds primarily formed between the phospho-nate groups and water mol-ecules. The latter constitute four of the corners of the MgO6 polyhedra and bind to the O atoms of the phospho-nate groups of neighbouring chains. The lattice water molecule is disordered around an inversion centre, exhibiting an occupancy of 0.25.