CeO2 based ceramics are currently the subject of extensive research interest as potential electrolyte materials for Solid Oxide Fuel Cells (SOFCs). They offer the advantage of lower operating temperatures than the current 'best' ceramic, yttria stabilized zirconia, but the onset of electronic conduction under the reducing conditions experienced on the fuel side of the SOFC is a potential drawback. A previous experiment on Polaris (RB 720272) studied the structural properties of pure CeO2-d under progressively more reducing atmospheres (i.e. increasing d). This continuation proposal requests 4 days beamtime on the same instrument to perform a complementary study of Y doped CeO2, to compare the nature of the oxygen vacancy defects formed by reduction of Ce4+ to Ce3+ with those induced by aliovalent (Y3+) doping.