The effect of the I'd addition method into the fresh Pd/(OSC + Al2O3) and (Pd + OSC)/Al2O3 catalysts (OSC material = CexZr1-xO2 mixed oxides) was investigated in this study. The CO + NO and CO + NO + O-2 model reactions were studied over fresh and aged catalysts. The differences in the fresh catalysts were insignificant compared to the aged catalysts. During the CO + NO reaction, only small differences were observed in the behaviour of the fresh catalysts. The light-off temperature of CO was about 20 degrees C lower for the fresh Pd/(OSC + Al2O3) catalyst than for the fresh (Pd + OSC)/Al2O3 catalyst during the CO + NO + O-2 reaction. For the aged catalysts lower NO reduction and CO oxidation activities were observed, as expected. Pd on OSC-containing alumina was more active than I'd on OSC material after the agings. The activity decline is due to a decrease in the number of active sites on the surface, which was observed as a larger I'd particle size for aged catalysts than for fresh catalysts. In addition, the oxygen storage capacity of the aged Pd/(OSC + Al2O3) catalyst was higher than that of the (Pd + OSC)/Al2O3 catalyst. (c) 2004 Elsevier B.V. All rights reserved.