[(PW11O39)(2)(Mo4S4O4(OH2)(2))](10-) anions were obtained through the stereospecific addition of the [Mo2S2O2](2+) oxothiocation to the monovacant alpha-[PW11O39](7-) anion. K-10[(PW11O39)(2)(Mo4S4O4(OH2)(2))](.)25H(2)O has been isolated as crystals and characterized by X-ray diffraction. The structure revealed a "sandwich-like" dimer of two alpha-[PW11O39](7-) subunits assembled by the noteworthy central cluster {H4Mo4S4O6}. The crystallization of the crude product produces an isomerically pure compound, which was characterized by P-31 and W-183 NMR. IR data were also supplied. In solution, the compound isomerizes, giving a second diastereoisomer. A kinetic experiment, carried out by P-31 NMR, allowed the conditions of the thermodynamic equilibrium to be determined. A structural relationship between the two isomers is proposed, fully consistent with NMR data. Cisoid and transoid isomers result in the relative disposition of each [PW11O39](7-) subunit, either staggered or eclipsed. An investigation of the formation of the [Mo2O2S2](2+) unit from the polycondensed cyclic precursor [Mo10S10O10(OH)(10)(H2O)(5)] and the aggregation process resulting in the oxothio [(PW11O39)(2)(Mo4S4O4(OH2)(2))](10-) compound has been undertaken. The studies were monitored by P-31 NMR and UV-vis spectroscopies. The reaction is quantitative in nearly stoichiometric conditions.