The effect of thermal degradation on the catalytic performance in light-off tests of Pd three-way catalysts supported on alumina and promoted with Ce-Zr mixed oxides has been investigated either under synthetic gas mixtures (for catalysts in powder form) or in engine tests (for catalyst monoliths). Accelerated aging of the systems by oven heating at 1273 K produces a significant sintering of both Pd and Ce-Zr mixed oxide active components, indicated by X-ray diffraction, transmission electron microscopy, and diffuse reflectance Fourier transform infrared (DRIFTS) techniques. In spite of these effects, the small differences between the light-off performances of fresh and aged systems show the relatively low relevance (particularly in real conditions under engine emissions) of the thermal degradation on the overall performance of the system. In situ DRIFTS results suggest that this is the result of the relatively low structural sensitivity of the desorption of hydrocarbon fragments adsorbed on the active sites, which act mainly as self-poisons for the catalytic reactions.