The enthalpies of the monoclinic to orthorhombic transition for a series of A2M3O12 (A = Al, Cr, Fe, In, and Sc; M = Mo or W) compounds were measured by differential scanning calorimetry, and entropies of transition were estimated. The enthalpies of formation from the binary oxides at 25 °C for several A2M3O12 samples were obtained from drop solution calorimetry in molten 3Na2O·4MoO3 at 702 °C. The monoclinic and orthorhombic phases of Sc2Mo3O12 and Sc2W3O12 are the only phases that are enthalpically stable under ambient conditions. The enthalpies of formation from the oxides (ΔHf,ox) for orthorhombic Sc2Mo3O12 and Sc2W3O12 are −47.2 ± 2.1 kJ/mol and −8.5 ± 2.7 kJ/mol, respectively. For Fe2Mo3O12, In2Mo3O12, and In2W3O12, ΔHf,ox values are 51.5 ± 4.5, 7.4 ± 2.9, and 44.5 ± 2.3 kJ/mol, respectively. These phases are entropically stabilized and/or metastable. Enthalpies of formation for phases that could not be measured by calorimetry have been estimated from the enthalpies of transition or trends in the enthalpies of formation. In general, the monoclinic phase is slightly enthalpically stabilized over the orthorhombic phase, while transition to the orthorhombic phase is entropically favored. This confirms that the orthorhombic phase is stable at high temperatures, the monoclinic is stable at low temperatures, and the monoclinic to orthorhombic transition is reversible.