Dispersed CeO2-ZrO2 is of interest as a thermally stable oxygen-storage component of automotive catalysts. Alumina-supported CeO2-ZrO2 samples were prepared by coimpregnation in order to maximize the interaction between Zr and Ce. The phases present, their particle sizes and the interactions among the phases of fresh, steam-aged and reduced samples were investigated by XRD and TEM. In the fresh samples, a particulate solid solution phase ZrxCe1-xO2 of cubic symmetry was identified. However, the zirconium concentration of this particulate phase was found to be smaller than that expected from the Zr loading. This suggests the existence of finely dispersed zirconia on the Al2O3 surface. For the steam-aged samples, a second Ce-Zr oxide solid solution phase of higher Zr concentration and tetragonal symmetry was found in addition to the original CeO2-based cubic solid solution. The appearance of this second phase may have resulted from sintering of the highly dispersed zirconia. The highly dispersed zirconia may also be responsible for preventing reaction between CeO2 and the Al2O3 support, since CeAlO3 was found only in high-temperature reduced samples without zirconia. The particle sizes of the various phases were measured by XRD and TEM. The particle size of the supported particulate phase decreased with increasing zirconium loading, but a discrepancy was noted between the XRD and the TEM results. This discrepancy is discussed in terms of compositional inhomogeneity in the ZrxCe1-xO2 solid solution phase.