Uranium fluoride gases are proposed as primary candidate fuels for ultrahigh-temperature gas core or vapor core reactor systems for a variety of space power applications. In these systems, the peak temperature of the fissioning gas can be as high as 5000 K and the inner wall temperature of the reactor cavity is within the range of 1000 to 2000 K. Two kinds of alumina, sapphire and polycrystal alpha alumina, and CaO partially stabilized zirconia are exposed to uranium hexafluoride gas in temperatures ranging from 973 to 1473 K and from 873 to 1073 K, respectively. Exposure tests are conducted in a UF6 flowing loop with an alumina reaction tube housed in a 1500 K electric-heated furnace.The reaction rates are measured using a discontinuous gravimetric method. The morphology of the exposed surfaces was observed by optical microscopy and scanning electron microscopy, and the reaction products were identified by X-ray diffraction and energy dispersive X-ray spectroscopy. Results indicate that alumina provides a relatively higher service temperature in UF6 environment. However, due to the highly reactive and chemically aggressive nature of UF6 at high temperatures, the maximum service temperature of alumina for a UF6-based gas core reactor is limited to 1273 K. Zirconia at temperatures above 973 K is not compatible with UF6.