Two series of Ni/MgO catalysts were prepared by reducing NiO/MgO samples of fixed Ni loading but different calcination temperatures and of varying Ni loadings but fixed calcination temperature. These catalysts were investigated in CO 2 reforming of methane under atmospheric pressure and characterized with XRD, TPR and H 2 -TPD techniques. A complete incorporation of NiO into the MgO "support" to form NiO-MgO solid solution during the calcination stage of the catalyst preparation was identified essential for the formation of stable Ni/MgO catalysts, and the presence of readily reducible "free" NiO in the calcined (unreduced) NiO/MgO samples was shown to produce the deactivating Ni/MgO catalysts during the CO 2 /CH 4 reaction. The reactivities of CO 2 /CH 4 were found sensitive to the particle size (or dispersion) of metallic Ni; the catalytic activity by CH 4 turnover frequency (TOF) decreased with increasing the Ni particle size. The reduced catalysts showed two H 2 -TPD peaks and the nickel sites corresponding to H 2 -TPD peak at higher temperature showed a higher activity than those associated with the peak at lower temperature. Our data demonstrate that the support in the stable catalysts was actually a kind of Ni x Mg 1-x O (x = 0.02-0.15) solid solution and the stable catalytic sites were associated with nanosized Ni particles (3-20 nm) in strong interaction with the solid solution support..