The phase relation in CeO2-Gd2O3 system has been established under slow cooled conditions from 1400 degreesC. Two phase regions, namely F-type cubic and C-type cubic, were observed in this system. A striking observation of this investigation is the stabilization of C-type gadolinia after Ce4+ substitution, which is attributed to decrease in average cationic size on Ce4+ substitution at Gd3+ site. The lattice thermal expansion behaviour of C-type gadolinia samples was investigated by High temperature-XRD. The lattice thermal expansion coefficient was found to gradually increase on increasing the Ce4+ content, within the C-type homogeneity range, in Ce1-xGdxO2-x/2 series. (C) 2004 Elsevier Ltd. All rights reserved.