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In this work, the reduction of Ce(IV) to Ce(III) in Ce0:9Zr0:1O2 (GS) and NiO(60wt%)/Ce0:9Zr0:1O2 (ZDC6) nanocatalysts, and the subsequent reoxidation, were studied by time-resolved in situ XANES technique. GS nanopowder was synthesized via the glycine-nitrate gel-combustion method. Nickel was incorporated via incipient impregnation with alcoholic solutions of Ni(NO3)2.6H2O with the appropriate concentration to get the final nominal composition. The in-situ XANES experiments were carried out at the LNLS, Campinas, Brasil, in the D06A-DXAS dispersive line equipped with a Si(111) monochromator and a CCD detector. The data were collected near the Absorption edges LIII of Ce and K of Ni, in the transmission mode. While collecting the transmitted signal, samples were heated at 10±C/min from 20±C to 800±C, with a 30 min isothermal period at 800±C, in a continuous °ow of Hydrogen (H2) (5mol%-He balance) or Oxygen (O2)(5mol%-He balance) in reduction or reoxidation experiments, respectively. In the reduction process, no significant diference in the Ce(IV) in GS and ZDC6 samples was observed up to 700°C, while complete reduction of NiO to Ni° was achieved at 700°C, Ce(IV)cations in ZDC6 sample started to reduce faster and deeper than in GS sample. This effect could be associated to the dissociation of H2 molecules over the Ni± formed,facilitating the reduction of Ce(IV) cations. During the reoxidation process, GS sample increases the reoxidization percentage almost linearly with temperature, while ZDC6 sample showed a two-step process: the ¯rst one from room tempera-ture to 300±C and the second one in the 600°C-800°C temperature range. From 300°C up to 600°C, the reduction percentage remained unchanged, while the main part of the reoxidation of Ni± to Ni(II) took place. Above 700°C and up to 800°C, the remaining Ce(III) and Ni± centers were completely reoxidized.