The [P2W18M6S6O74(H2O)(6)](12-) anions, M = Mo, W, were obtained through the stereospecific addition of the dithiocation [M2O2S2](2+) to the trivacant A-alpha-[PW9O34](9-). K-12[P2W18Mo6S6O74(H2O)(6)]. 26H(2)O has been isolated as crystals and has been characterized by X-ray diffraction (orthorhombic Pmn2(I) with a = 31.530(6) Angstrom, b = 19.703(4) Angstrom, c = 18.761(4) Angstrom, Z = 4). The structure of the anion consists of a sandwich-like arrangement of two alpha-[PW9O34](9-) subunits bridged by three [Mo2O2S2(H2O)(2)] cores. The X-ray diffraction structural analysis showed that one [Mo2O2S2] bridging unit was rotated through an angle of 180 degrees with respect to the other two. Among the six water molecules attached to the Mo centers, four are directed toward the inner cavity while the two remaining ones are directed out of the cavity. According to the X-ray data, W-183 NMR characterizations of both compounds show the lowering of the local symmetry of the alpha-[PW9O34](9-) subunit from C-3v to C-s. For [P2W24S6O74(H2O)(6)](12-) two additional deshielded resonances were observed characteristic of the distribution of the three [W2O2S2(H2O)(2)] bridging units. Infrared data are also given.