Dimanganese-substituted gamma-Keggin heteropoly tungstates have been synthesized by reaction of the lacunary species gamma-[(SiO4)W10O32](8-) with appropriate mixtures of Mn-II and MnO4-. The crystal structure of [(CH3)(3)(C6H5)N](4)[(SiO4)(W10Mn2O36H6)-O-III]. H2O (anion 1) was determined by X-ray diffraction. Crystallographic data : space group P (1) over bar, a = 12.951(3) Angstrom, b = 14.429(3) Angstrom, c = 20.347(4) Angstrom, alpha = 81.95(3)degrees, beta = 88.92(3)degrees, gamma = 67.48(3)degrees, V = 3475.2(13) Angstrom(3), and Z = 2. The final R value is 7.29% for 15861 reflections with I > 2 sigma(I). The anion has the anticipated gamma-Keggin structure with virtual C-2v symmetry. The two Mn cations occupy adjacent, edge-shared octahedra with bridging hydroxo and terminal aqua ligands. Anion 1 can be oxidized and reduced to the corresponding (MnMnIV)-Mn-III (2) and Mn-2(II) (3) species respectively. The magnetic susceptibility of 1 between 2 and 300 K indicates that the Mn-III cations are antiferromagnetically coupled, with J = -17.0 cm(-1) and g = 1.965. No simple magnetic behavior was observed for 2 or 3.