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Optically active (-) 589 ethyl ( S )-2-phenylbutyl thioether, (-)( S ) C -Et(PhBu)S ( I ), and its new diastereoisomeric mercury (II) chloride adduct, 1:2, (-)[( S ) S ( S ) C -Et(PhBu)S.(HgCl 2 ) 2 ] 2 , ( II ) were stereoselectively synthesized; the absorbance (UV) and circular dichroism (CD) spectra were measured and the crystal and molecular structure of complex ( II ) was determined by single-crystal X-ray diffraction. Two different Hg centres are present whose coordination environments are built by two short bonds to chloride ligands in one case, and to one chloride and one sulphur in the other one. These originate digonal units. Electroneutrality is achieved by a further chlorine, which can be considered prevalently ionic and bonded to the two Hg centres, forming square bridging systems nearly perpendicular to the digonal molecules. The coordination polyhedra can be interpreted as 2 + 4 tetragonally-compressed octahedra with the four longer contacts lying in the equatorial plane. IR spectroscopic data are consistent with the presence of one bent and one linear Cl-Hg-Cl moiety. The absolute configurations at both stereogenic centres of the formed diastereoisomeric complex ( II ) are ( S ). The ( S ) S absolute configuration at the stereogenic sulphur atom bonded to the mercury(II) atom in complex ( II ) has been related with the negative Cotton effect assigned in its circular dichroism (CD) spectrum to a charge-transfer transition at ca. 230 nm. The stereoselective oxidation of ( I ) and ( II ) with hydrogen peroxide, induced by the stereogenic carbon atom ( S ) C of the enantiopure sulphide, gave (-) 598 ethyl ( S ) C -2-phenylbutyl( S ) S -sulphoxide, (-) 598 [( S ) S ( S ) C -Et(PhBu)SO], ( III ), having 18.1% de. Oxidations carried out in the presence of a 200 molar excess of mercury(II) chloride gave (-) 598 ethyl ( S ) C -2-phenylbutyl( R ) S -sulphoxide, (-) 598 [( R ) S ( S ) C -Et(PhBu)SO], ( IV ) with 31% de, showing the cooperative influence of mercury(II) chloride on the selectivity of the oxidation reaction.