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In this work, the reduction properties of ceria (CeO2) used as support of metallic nanoparticles (Au, Pd, Au0.9Pd0.1, and Au0.8Pt0.2) were elucidated. The catalysts were exposed to a reduction treatment in H2atmosphere at 500 °C. In situX-ray absorption spectroscopy (in situXAS) and in situtime-resolved XAS measurements at the Ce L3edge were used to probe the local atomic order around Ce atoms and the Ce oxidation state. In this way, it was observed that the supported metallic nanoparticles improve the reduction process of the support. Moreover, the reduction of ceria is dependent on the composition of the metallic nanoparticle supported. By means of in situXAS measurements at the Au and Pt L2,3edges, it was possible to obtain information concerning the fractional change in the number of 5d-band electron holes relative to a reference material for the Au- and Pt-containing nanoparticles. In this way, it was observed as evidence for the charge transfer effect from the nanoparticles to the support, which is responsible for the improved reduction of the CeO2support in the presence of nanoparticles. This result is corroborated by the observation of energy shifts on the Au 4f, Pd 3d, and Pt 4f binding energy values of the measured X-ray photoelectron spectroscopy (XPS) spectra. In this way, this work contributes to elucidating the physical mechanisms responsible for the enhanced support reduction effect existing in modern ceria-based catalyst.