The study deals with the identification of the active nickel phase as a function of the oxygen conversion during the partial oxidation of methane to synthesis gas by means of in situ high-temperature X-ray diffraction (HTXRD), semi-in situ X-ray photoelectron spectroscopy (XPS) on a nonporous model catalyst (wafer), activity measurements, and temperature-programmed surface reaction (TPSR) experiments. The results demonstrate that the combustion of methane partly proceeds on bulk nickel oxide. A highly active phase, which is associated with partially oxidized nickel, is suggested to account for the major part of the combustion reaction. Finally, the reforming of methane to synthesis gas calls for the presence of metallic nickel sites. Evidently, the nature of the active phase is not uniform along the catalyst bed under reaction conditions. This highly complicating factor should be taken into account when investigating the mechanism of the partial oxidation of methane to synthesis gas. It is concluded that the results of mechanistic studies conducted at high oxygen conversion levels using either a fluidized bed reactor or a pulse apparatus should be considered with care.