A NiO/MgO catalyst prepared by impregnation, which reduced in H2 had very high CO yield and stability in CO2 reforming of methane, was investigated by XPS, XRD, BET and pulse-MS response. This catalyst was compared to that obtained by mechanical mixing of powders of the two oxides. It was found that the entire NiO formed a solid solution with MgO in the former catalyst, while only a fraction of NiO formed a solid solution with MgO in the latter one. BET revealed that, in contrast to NiO and MgO, the NiO/MgO catalyst prepared by impregnation had a high stability to sintering, because its surface area hardly changed during calcination from 1.5 to 20 h at 800°C. In the same catalyst, a surface enrichment in MgO, which was greater after than before reduction, was detected. Compared to MgO or NiO, this catalyst had a lower Mg(2p) and a higher Ni(2p3/2) binding energy. This indicates that electron transfer from NiO to MgO took place, which, increasing the binding between the two oxides, might be responsible for the resistance of the solid solution to sintering. Because of the interactions between Ni and Mg, the clustering of Ni, which stimulates carbon deposition is inhibited. This explains the high stability of the CO yield in the CO2 reforming of methane over the NiO/MgO catalyst prepared by impregnation. The pulse-MS response suggested that the decompositions of CO2 to CO and O and of CH4 to C and H are involved in the reaction mechanism of CO2 reforming of methane over the reduced NiO-MgO solid solution catalyst.