An unprecedented solvothermal treatment of triangular molybdenum oxo clusters K2[Mo3O4(Hnta)3] (1, nta = nitrilotriacetate) and H2bipy[Mo3O4(C2O4)3(H2O)3] (2, bipy = 4,4′-bipyridine) was successfully established to prepare a series of new oligonuclear Mo(IV–VI) oxo clusters 3–5. Hydrothermal treatment of 1 in aqueous solution produced the hexanuclear cluster [MoIV6O10(bpy)4(Hnta)2]·10H2O (3, bpy = 2,2′-bipyridine). Solvothermal oxidation of 2 was performed in pyridine (py) to yield the unprecedented decanuclear mixed-valent oxo cluster [MoIV6MoVI4O24py8]·py (4). The further oxidized product [MoV8MoVI2O26py8]·2py (5) was obtained from a reaction at a higher temperature and for a longer duration. Clusters 3–5 have been characterized by X-ray crystallography, elemental analysis, and UV/VIS and IR spectroscopy.