High-purity monoclinic ZrO2 and monoclinic Nb2O5 powder samples were mixed in varying ratios, and heated at 1300 degrees C for 24 h before furnace cooling. X-ray diffraction analysis showed that two compounds, Nb2O5 . 6ZrO(2) and Nb2O5 . 8ZrO(2) (or (Zr, Nb)(8)O-17 and (Zr, Nb)(10)O-21), were formed. Some of the reflections from the two compounds were overlapped and interfered with those from the zirconium oxide, especially the tetragonal ZrO2. A thermodynamic analysis was used to demonstrate that it is possible to have ZrO2 as well as Nb2O5 formed on Zr-Nb alloys in an oxidizing environment. The relevance of these results (experimental and calculated) to the corrosion of a Zr-Nb alloy in high temperature aqueous environments is discussed. It is suggested that the incorporation of niobium into the oxide during the corrosion of Zr-Nb alloys could be by the formation of com pounds such as (Zr, Nb)(8)O-17 and (Zr, Nb)(10)O-21. Also, the tetragonal ZrO2, which has been found to be a barrier layer oxide, could, in fact, be either of the ternary compounds Nb2O5 . 6ZrO(2), and Nb2O5 . 8ZrO(2) both of which are orthorhombic with lattice parameters very close to those of the tetragonal ZrO2.