The corrosion behaviour of Y2O3 (yttria) at 900 degrees C in flowing UF6 (uranium hexafluoride) was investigated under a pressure of 1.33 x 10(-3) Pa. A test loop was used to study the chemical reaction of sintered and hot-pressed yttria samples with flowing UF6 gas in an alumina reaction tube installed in an electrically heated horizontal 1200 degrees C type furnace. A weight increase was observed after each exposure testing. Scanning electron microscopy (SEM) examination of the exposed samples identified two layers of reaction products. Significant cracking and porosity were observed in the outer layer after the experiment while the inner layer thickness decreased with increasing the exposure time. Thermodynamical analysis using Facility for the Analysis of Chemical Thermodynamics (FACT) software packages has been performed and relevant interface reactions have been suggested. Pilling-Bedworth Ratios for both layers were calculated and found to be in agreement with the morphology of the layers. Electron microprobe (EMP) and X-ray diffraction analysis (XRD) revealed the formation of UO2 and YF3 in the outer layer and a possible formation of YOF in the inner layer. The formation of the outer layer was primarily due to oxygen-fluorine exchange reactions between yttria and uranium fluorides while the inner layer was a direct consequence of the fluorine inward diffusion and reaction with yttria. (C) 1998 Elsevier Science B.V. All rights reserved.