Variation of lattice constants of Ce1-xMxO2-0.5x-delta (M=Gd, Sm, Y; x=0, 0.1, 0.2) with temperature and oxygen ion vacancy concentration was investigated with X-ray diffraction (XRD) at 600-1000degreesC under various gas atmospheres. Linear thermal expansion and increase of expansion coefficient with trivalent cation content were observed in every specimen under N-2 and O-2 atmospheres. In N-2/H-2/H2O gas mixtures, expansion due to generation of oxygen ion vacancies was observed at temperatures higher than 600degreesC for every specimen. Irrespective of the kinds of trivalent cations, the dependence of lattice constants on delta was almost linear for the specimen with x=0.2, whereas bending point was observed for those with x=0.1 and for pure ceria. The bending point was observed at around 0.1 of 0.5x+delta, which was the total amount of oxygen ion vacancies. (C) 2004 The Electrochemical Society.