Mg-promotion of natural clay based Ni-catalysts was considered, as a way of boosting the dry reforming of methane (DRM) activity of these materials. The results of the DRM experiments performed at temperatures from 600 C to 850 C evidenced much higher methane and CO 2 conversions for the Mg-promoted catalysts. Mg-promotion led of course to a significant increase of CO 2-adsorption ability (basicity). However, the increased catalytic activity of the Mg-promoted materials was rather linked to increased Ni-dispersion and Ni 0 crystallite size. Indeed, independent of the physico-chemical properties of the support, the presence of Mg led to the formation of a MgNiO 2 mixed phase that, upon reduction, resulted in the formation of metallic Ni clusters having sizes around 7–9 nm, considerably smaller than in any of the non-promoted catalysts. Carbon formation was found to take place to a greater extent in the presence of the Mg-promoted catalysts, due to C–H bond activation leading also to favored direct methane decomposition (DMD). In spite of this, the activity of the Mg-promoted catalysts was well maintained over 5 hour DRM experiments performed at 750 C.