Pt, Rh, and bimetallic Pt-Rh catalysts with pure alumina and Ce-ZrO2-modified alumina washcoats are studied by XPS and TEM. The synergistic effect of platinum and rhodium is shown to be capable of stabilizing rhodium against the formation of a reduction-resistive oxide phase on alumina-supported catalysts in high-temperature air aging. Rhodium is found to exist in two different states: reduction-resistive rhodium oxide in strong interaction with alumina support and reducible rhodium oxide on the surface of Pt-Rh bimetallic particles. The growth of the noble metal particle size in the aging treatment is observed to be more pronounced on the Ce-ZrO2-containing catalysts than on pure alumina-supported catalysts. Moreover, further increase of the particle size is detected on the Ce-ZrO2-containing catalysts after H-2 reduction at 300 degreesC. This may be related to the ability of Ce-ZrO2 to release oxygen in reducing atmosphere. (C) 2004 Elsevier Inc. All rights reserved.