The transformation of perovskite to an oxide-supported metal during reaction of partial oxidation of methane (POM) has been very often cited in literature as the “reduction” of the perovskite, without any detailed mechanism. In this work, the in situ transformation of initial LaNiO3 perovskite to Ni0/La2O3 was studied under flowing CH4/Ar and CH4 + O2/Ar (POM mixture) in temperature-programmed conditions. The catalyst was characterized before, during and after these experiments using X-ray diffraction (XRD), thermogravimetric analysis (TGA) and high resolution transmission electron microscopy (HRTEM/EDS) with Fast Fourier Transform (FFT) and Reverse FFT. Total oxidation of methane over LaNiO3 and lanthanum oxide (La2O3), as well as formation of syngas over the resulting Ni0/La2O3 catalysts, were studied for interpreting the three-step evolution of LaNiO3 to Ni0/La2O3, and evidencing NiO demixing from perovskite. A new global kinetic model of POM process over the final bifunctional Ni0/La2O3 catalyst was described.