Sodium dihydrogen phosphate (NaH2PO4) is monoclinic, space group P21/c, witha= 6.808 (2),b= 13.491 (3),c= 7.331 (2) Å, β = 92.88 (3)°;Z= 8. The structure was solved and refined anisotropically (R= 0.028) with 1404 reflexions measured on a diffractometer (CuKα radiation). Corrections for absorption, secondary extinction and anomalous scattering were applied. The four independent hydrogen atoms were located on a difference map; they are involved in four O … O hydrogen bonds of length 2.654 (2), 2.589 (2), 2.559 (2), and 2.500 (2) Å. The short contact (2.500 Å), which is not astride any symmetry element, is of the type O‐H … O (asymmetrical). The distorted coordination polyhedra of the sodium atoms are discussed in the light of recent theories on electrostatic balance. Double layers of sodium polyhedra sharing edges, parallel to (010), can be observed in th