A Pd/(Ce,Zr)O-x/Al2O3 catalyst has been studied to determine how thermal aging under air at 1273 K influences its structural characteristics and catalytic properties for CO oxidation reactions under stoichiometric conditions, employing either O-2 or an NO-O-2 mixture as oxidants. XRD and TEM-EDS show that significant sintering of both Pd and Ce-Zr mixed oxide active components is produced upon aging. As a consequence, lower CO oxidation or NO reduction activities were observed in catalytic tests. In situ DRIFTS studies for both reactions indicate that the lower activity of the aged systems is not only due to the sintering of active components but also due to a consequent loss of (Ce, Zr)O-x-induced promoting effects on Pd activity.