CO2 methanation was studied over Ni-Mg-USY zeolite catalysts. Mg was added by incipient wetness impregnation and ion exchange. Whatever the Mg incorporation method used, Mg is able to improve the performance of the Ni-zeolite catalysts. Magnesium impregnation at lower contents (below 2.5%) over a 5%Ni/zeolite leads to an enhancement of the CO2 conversion into methane of 15% at 350-450 degrees C, probably due to the induced increase of the Ni particle dispersion, as well as to the possible activation of CO2 on the defects present on the MgO surface. At higher Mg contents, the stronger interaction between Ni and Mg oxides, leading to the formation of NiO-MgO solid solutions, seems to reduce the reducibility of the Ni species, decreasing the amount of nickel active sites available and thus the catalytic performance. Furthermore, important improvements of the catalytic performance (CO2 conversion and CH4 selectivity increased around 20% at 350-450 degrees C) were also found for the Mg-exchanged 5%Ni/zeolite. The results are comparable to those obtained by impregnation, but lower Mg contents are required. The enhanced activation of CO2 on Mg2+, the more dispersed Ni particles on the support, as well as the enhanced Ni reducibility when compared to the impregnated catalysts can explain the observed results. The beneficial effect of the Mg addition both by impregnation and ion exchange was also observed for samples containing higher amounts of Ni, the best sample remaining stable after 10 h under reaction conditions. Thus, Mg could be an interesting promoter for CO2 methanation over Ni-zeolite catalysts.