Using first‐principles calculations, the enthalpies of formation of NdAlO 3 and Nd 4 Al 2 O 9 from neodymia and alumina are predicted to be −41.1 and −104.5 kJ/mol‐form, respectively, and Nd 4 Al 2 O 9 is predicted to be a stable compound at all temperatures up to its peritectic decomposition temperature. In the case of NdAlO 3 , where reported experimental enthalpies of formation differ, our predicted value is used to determine which experimental value is more accurate. The enthalpies of formation calculated by the first‐principles method are combined with experimental phase equilibria to evaluate the Gibbs energies of phases in the Al 2 O 3 –Nd 2 O 3 system. The resulting Gibbs energy functions are compared with previous modelings of the system and are shown to be an improvement as more thermochemical data have been included. The combined first‐principles/thermodynamic modeling approach employed in the current work shows promise in enabling the evaluation of important ceramic systems in which experimental thermochemical data are scarce.