The corrosion mechanism and microstructural characteristics of yttria (Y2O3) exposed to uranium hexafluoride (UF4) and uranium tetrafluoride gases at temperatures above 900-degrees-C were investigated. Processed yttria samples were exposed for times ranging from 5 to 40 min. A weight increase was observed after each test exposure. An outer corrosion scale composed of UO2 and YF3, and an inner corrosion layer of YF3, were observed following the UF6 reaction at nearly 1200 K. During the UF4 exposure, an outer layer containing a liquid mixture of fluoride, oxygen, yttrium, uranium compounds, a secondary reaction layer of UO2 followed by an inner layer composed of a solution of Y2O3 and YF3, possibly in the form of YOF inner layer, were observed. It was found that the corrosion process is controlled by the diffusion which is evidenced by the parabolic growth rate of the secondary layer. After completion of the solidification process, the formation of a eutectic region and dendrites were observed throughout the outer scale. In general, at temperatures above 1173 K, high corrosion rates were observed, and yttria did not appear to be a viable containment material for UF4 and UF6.