The influence of the Mg addition on the catalytic performance of a 20% Ni based SiO2/silica cloth system in the decomposition of methane for making "COx-free" H-2 in the T-R range 773-873 K has been evaluated. As the Mg-at/Ni-at ratio increases from 0.10 to 2.20 the incipient Ni-Mg-O interaction becomes stronger until the formation of Ni-x Mg(1-x)O solid solution. The presence of Mg2+ ions allows a higher carbon capacity (C/Ni, number of CH4 molecules converted per Ni atom until complete catalyst deactivation) and consequently a higher H-2 productivity of the Ni based systems which results in a smoothing effect of the unavoidable declining activity trend even if it does not affect the tip growth coking mechanism.