The aim of this paper is to study the oxidation states of metal cations and the changes in the crystalline phases in NiO/Ce0.9Zr0.1O2 nanocatalysts under typical conditions of methane catalytic partial oxidation reaction by in situ X ray absorption near edge spectroscopy (XANES) and X-ray powder diffraction (XPD) studies. The Ce0.9Zr0.3O2 mixed oxide was obtained by glycine-nitrate-combustion method, being the nickel incorporated by incipient wetness impregnation. The evolution of the crystalline structure with temperature and operating conditions was followed by in-situ XPD experiments and the oxidation states of Ce and Ni cations by in-situ XANES experiments at the Ce-L-III and Ni-K absorption edges. It was observed that NiO is completely reduced to ;Ni at temperatures above 650 degrees C while Ce0.9Zr0.1O2 fluorite-like phase showed changes in lattice parameter due to cerium reduction and crystallite growth, but no phase transformations or segregations were observed. It was confirmed that Ni is the active centre to activate methane molecule while Ce4+/Ce3+ ratio is strongly related with CO/CO2 concentration in the exhaust gas flow.