The thermodynamic stability of Th-doped yttrium iron garnet (Y 3 Fe 5 O 12 , YIG) as a possible actinide-bearing material has been investigated using calorimetric measurements and first-principles electronic-structure calculations. Yttrium iron garnet with thorium substitution ranging from 0.04 to 0.07 atoms per formula unit (Y 3Àx Th x Fe 5 O 12 , x ¼ 0.04–0.07) was synthesized using a citrate–nitrate combustion method. High-temperature oxide melt solution calorimetry was used to determine their enthalpy of formation. The thermodynamic analysis demonstrates that, although the substitution enthalpy is slightly endothermic, an entropic driving force for the substitution of Th for Y leads to a near-zero change in the Gibbs free energy. First-principles calculations within the density functional theory (DFT) indicate that the main limiting factors for Th incorporation into the YIG structure are the narrow stability domain of the host YIG and the formation of ThO 2 as a secondary phase. Nevertheless, the defect formation energy calculations suggest that by carefully tuning the atomic and electronic chemical potentials, Th can be incorporated into YIG. The thermodynamic results, as a whole, support the possible use of garnet phases as nuclear waste forms; however, this will require careful consideration of the repository conditions.