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The phase relations in the system U02-U03-Yz03, particularly in the Y203-rich region, were examined by X-ray and chemical analyses of reacted powders heated at temperatures up to 1700°C in H2, C02-CO, and air. Four phases were identified in the system at temperatures between 1000" and 1700°C : U308, face-centered cubic solid solution, body-centered cubic solid solution, and a rhombohedral phase of composition (U,Y)70,z ranging from 52.5 to 75 mole yo Yz03. The rhombohedral phase oxidized to a second rhombohedral phase with a nominal composition (U,Y),015 at temperatures below 1000°C. This phase transformed to a face-centered cubic phase after heating in air above 1000°C. The solubility of UO, in the body-centered cubic phase is about 14 mole yo between 1000" and 1700°C but decreases to zero as the uranium approaches the hexavalent oxidation state. The solubility of Y z 0 3 in the face-centered cubic solid solution ranges from 0 to 50 mole yo Y z O~ under reducing conditions and from 33 to 60 mole yo Y203 under oxidizing conditions at 1000°C. At temperatures above 1000°C, the face-centered cubic solid solution is limited by a filled fluorite lattice of composition (U,Y)O2. For low-yttria content, oxidation at low temperatures ( <3OO0C) permits additional oxygen to be retained in the structure to a composition approaching (U,Y) 02.z~. A tentative ternary phase diagram for the system U02-U03-Y203 is presented and the change in lattice parameter and in cell volume for the solid-solution phases is correlated with the composition.