Various CeOx-Al2O3 powders were prepared and the influence of the method of support preparation on the thermostability of the mixed alumina-supported Pt catalysts and on their oxidation of CO was investigated. It was found that the complexing agent-assisted sol-gel method is effective in improving the thermostability of the mixed alumina-supported Pt catalysts. Monolith Pt-Rh three-way catalysts were prepared using these aluminas and other thermostable alumina including Pa. Activity of the catalysts was also investigated after conducting accelerated thermal engine aging in order to test their durability. Under normal operating conditions of the automobile engine, the activity of the Pt-Rh three-way catalyst using the CeOx-Al2O3 support prepared by the complexing agent-assisted sol-gel method is nearly equal to that of the catalyst using the CeOx impregnated onto BaO-Al2O3 powder prepared by the same sol-gel method.