Reduction of iron oxide/CeO₂solid due to methane anaerobic combustion has been studied usingin situtime-resolved synchrotron X-ray powder diffraction and mass spectrometry measurements. All the chemical reactions involved in the process have been mapped. Rietveld profile fitting analysis has given a quantitative estimation of the different phases at every step of the reaction. The reaction temperature has been estimated using thermal expansion of both Fe₂O₃and CeO₂. From the structural parameters of CeO₂it has been possible to claim that CeO₂participates in the reduction process by transforming into a reduced CeO_xform. The number,x, of O atoms per formula unit has been estimated by measuring the percentage variation of the unit-cell parameters.