Phase equilibria in the Nd 2O 3 MoO 2 MoO 3 system have been investigated at 1273, 1473, and 1673°K under a controlled gaseous atmosphere with emphasis on the formation and stability of ternary phases. New fluorite-related compounds of molybdenum VI and V have been characterized: Nd 6MoO 12, Nd 10Mo 2O 21, and Nd 3MoO 7. In the MoO 2-rich portion of the phase diagram a new cubic-related molybdenum IV compound, Nd 2Mo 3O 9, has been found. On the basis of phase equilibria at 1473°K, the standard free energies of formation of Nd 2MoO 5, Nd 3MoO 7, Nd 6MoO 12, and Nd 2MoO 6 were determined from metallic molybdenum, Nd 2O 3, and oxygen. The role of the rare earth element in selective oxidation of molybdenum and crystal chemical properties of the ternary compounds is discussed in relationship to the neighboring systems of praseodymium and samarium oxides.