The effect of Zr, Si, La, and Pa on the thermal aging of PI, Ph, and Ce in automotive exhaust gas catalysts supported on metal foils was studied with X-ray photoelectron spectroscopy (XPS). Platinum was in metallic state under all circumstances studied in this work. Rhodium was partially (about 30%) oxidized already at room temperature. The decreases in the binding energies of the reduced noble metals due to aging was explained in terms of particle growth. The binding energy of rhodium oxide increased during aging indicating rhodium diffusion into alumina. Cerium was oxidized during aging. It was found that in the lanthanum containing catalysts the binding energies of platinum and rhodium were below the values for bulk metals, and the degree of oxidation of cerium was lower than in the other samples. It was concluded that the presence of lanthanum facilitated the reduction of the noble metals and cerium.