Ni(15%)/MgO and Co(12%)/MgO have been used as catalysts to carry out the oxidation of methane to synthesis gas in a fluidized-bed reactor. The fresh and used catalysts have been characterized through a battery of instrumental techniques aiming at finding the relationship among the operating conditions, activity, and stability of the reacting system. The fresh catalysts are made up of a mixture of crystalline MgO and MgMO(2) (M = Ni or Co) while the used ones also contain NiO and CoO. This explains the on-stream activation of the solid in terms of an in situ reduction of the oxides to generate the metallic centers needed to catalyze the reforming reaction. Under some of the experimental conditions employed, the SEM micrographs of the used solids show coke formation in whiskers. These will be burnt in the oxidizing zone of the fluidized bed, thus releasing Ni particles, which may be deposited on the freeboard walls. These metallic particles can catalyze back reactions, decreasing the overall yield to the desired reaction products, CO and H-2.