X-ray crystal-structure analysis of basic mercury(II) cyanide, known under the formula HgO · Hg(CN) 2 , showed that the compound is a bis -cyanomercuric oxide. It is orthorhombic holosymmetric with the space group D 2h 16 – Pnam . The unit cell with a = 18.93, b = 7.09 and c = 3.90 Å contains four formula units. The parameters of all atoms were obtained by means of Fourier and difference Fourier projections on (001) and from symmetry conditions of the space group. The structure consists of planar (NCHg) 2 O molecules the shape of which is shown in Fig. 2. Intra- and intermolecular distances as well as bond angles are given in Table 3. The molecules, related symmetrically by [001] screw axes, are linked in vertical columns through the shortest intermolecular approaches of 2.53 Å between Hg I and O atoms. With the exceptions of the vertical separations of Hg II –N I 2.84 Å and Hg II –N II 2.96 Å which ensure the connection of columns, all the other intermolecular shortest distances correspond to the sum of van der Waals radii. The coordination polyhedra around Hg I and Hg II are deformed tetrahedra and octahedra respectively, with the bivalency of Hg II being more accentuated than in the case of Hg I .