Thermal decomposition of the mercuric arenesulphonate dihydrates Hg(O3SR)2,- 2H20 (R = C6X5, p-HC6X4, or m-HC6Cl4; X = Cl or F) at c. 130-240° gave the corresponding diarylmercurials, the polyhalogenobenzenes RH, and sulphur trioxide (or sulphuric acid) in all cases, together with RSO3H (R = C6F5 or p-HC6F4), and p-(p-HC6F4SO3Hg)2C6F4. By contrast, decomposition of Hg(O3SR)2, 2H2O (R = m-HC6F4 or o-HC6X4) gave the corresponding (-HgC6X4SO3-)n derivatives, sulphonic acids, polyhalogenobenzenes, and sulphur trioxide in all cases, together with m-HC6F4HgO3S-m-HC6F4 and o-(o-HC6Cl4SO3Hg)2C6Cl4. The compounds RHgO3SR (R = C6Cl5, isolated as the monopyridinate, or p-HC6Cl4) were obtained from decomposition of the appropriate mercuric sulphonates at c. 165°. Identities of (-HgC6X4SO3-)n derivatives were established mainly by cleavage with triiodide ions in N, N-dimethylformamide giving the salts M(IC6X, SO3) (M = Na or S-benzylthiouronium), and of (HC6X4SO3Hg)2C6X4 derivatives by similar degradation giving I2C6X4 and M(HC6X4SO3) (M = Na or Ph4As). Similar degradation of C6Cl5HgO3SC6Cl5, py, p-HC6Cl4HgO3S-p-HC6Cl4, and the known mercurials C6Cl5HgCl, (p-HC6Cl4)2Hg, and (o-HC6Cl4)2Hg gave the corresponding iodopolychlorobenzenes. The mercurated derivative (-o-HgC6F4SO3-)n gave (-o-HgC6F4-)3 on thermal decomposition, and crystallization from water yielded (-o-HgC6F4SO3-)n, 2nH2O, which was converted into (Ph4As)(o-ClHgC6F4SO3) by tetraphenylarsonium chloride. The mercuric sulphonates were prepared from mercuric acetate and the appropriate sulphonic acids in acetic acid (X = Cl) or water (X = F). Sulphonic acids, obtained by sulphonation reactions, were characterized as the dihydrates and in some cases sodium salts (polychloro derivatives) or as barium and tetraphenylarsonium salts (tetrafluoro derivatives).