1,6-DITHIAcycloDECA-cis-3, cis-8-diene1 forms a bis-(mercuric chloride) adduct. The obvious postulate for the structure of this complex is shown in diagram I, with each mercury atom in three-fold co-ordination. The actual crystal structure determined by X-ray analysis, however, is of the polymeric type (diagram II), with one mercury atom, Hg(2), in tetrahedral co-ordination and the other, Hg(l), in an only slightly distorted mercuric chloride molecule. The Hg(2)-S and Hg(2)-Cl distances are 2.53 and 2.51 Å, respectively. The complex crystallizes in the monoclinic system, space group P21/m, with two molecules of C8H12S2Hg2Cl4 in a cell of dimensions a = 7.29, b = 17.01, c = 6.20 Å, β = 92° 43'. The structure was solved by Patterson and Fourier methods and refined by least-squares calculations; the final value of the discrepancy R over 1,158 independent reflexions is 13.6 per cent.