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Neutralization of 5-chloro-phenylene-1,3-diphosphonic acid by sodium hydroxide in water affords quantitative yields of a tetrasodium salt as a crystalline decahydrate. The structure consists of thick uni-dimensional gutter-shaped domains in which the sodium cations are embedded in a hydrate matrix. The edges of the gutters are supported by indented stacks of anions. Within the domains of hydrated cations of the composition [Na 4 (H 2 O) 10 ] 4+ the inequivalent sodium cations reside in the centers of vertex-, edge- and face-sharing polyhedra, of which only one has phosphonate oxygen atoms and a chloro-substituent of the anions as donor atoms. The Cl-Na contacts and an extended hydrogen-bonding network involving all water molecules and the phosphonate groups are connecting the gutters into space-filling bundles extending along the crystallographic c -axis. The asymmetric units of the gutter-like realms of the cations are chiral, but both enantiomeric forms are present in the crystal and related through centers of inversion and glide planes. [ABSTRACT FROM AUTHOR]