CoNa(H2PO3) 3.H20, M r : 342.88, orthorhombic, Pbca, a = 9.054 (2), b = 14.706 (3), c = 14.759 (3) A, V--1965.1 (7) A 3, Z = 8, D x = 2.32, D m = 2.30 Mg m -3,/z(Mo Ka) = 2.38 mm-l; R = 0.046 (R w = 0.072) for 1980 observed counter reflections. The structure contains three crystallographically non-equivalent HPO a tetrahedra. The hydrogen bonds give rise to a three-dimensional network in which the Co and Na atoms are surrounded by octahedra of O atoms. min -1. The scanning interval was 1 °. 162 reflections were classified as unobserved [I < 1.96trl(I), where 31(I ) was derived from counting statistics]. Data were corrected for Lorentz and polarization effects and for absorption (pr --0.83). Transmission factors varied between 0.30 and 0.32. The density was determined pycnometrically in xylene at 298 K.