The phase relations in the system CeO 2 –HfO 2 have been studied under slow cooled conditions. About 10mol% of HfO 2 was found to be soluble in the CeO 2 lattice under these conditions. All the other compositions were found to be biphasic mixtures of F-type cubic and monoclinic phases. The lattice thermal expansion behavior was also investigated for a few single phasic as well as biphasic compositions, by means of high temperature XRD. It was found that the compositions with a higher amount of HfO 2 in CeO 2 lattice showed higher thermal expansion behavior.