CeO2–ZrO2–γ-Al2O3 mixed oxides with different molar compositions have been synthesized by the sol-gel method using nitrate–alkoxide precursors. Some relationships between the molar composition of the ternary systems and their textural and structural properties are presented. CeO2–ZrO2–γ-Al2O3 mixed oxides have been studied and characterized using x-ray diffraction, transmission electron microscopy, and Raman spectroscopic analyses. By complete porosity analyses of all samples before and after hydrothermal (up to 75% steam) treatment at 500 °C, we have been able to optimize a highly hydrothermally stable CeO2–ZrO2–γ-Al2O3 system with a molar ratio of 10:10:80 mol%. γ-Al2O3 alone and CeO2-doped as well as ZrO2-doped γ-Al2O3 were also investigated by the hydrothermal treatment, and their steam-sensitive properties have been compared with those of the ternary CeO2–ZrO2–γ-Al2O3 system.