The effects of nickel loading, calcination temperature, support, and basic additives on Ni-based catalyst structure and reactivity for CH4 reforming with CO2 were investigated. The results show that the structure of the nickel active phase strongly depends on the interactions of the metal and the support, which are related to the support properties, the additives and the preparation conditions. "Free" Ni species can be formed when the interaction is weak and their mobility makes them easily deactivated by coking and sintering. The effect of strong metal-support interaction (SMSI effect) is different for various supports. The formation of solid solution of Ni-Mg-O-2 and the blocking of TiOx by the partially reduced TiO2 can both decrease the availability of Ni active sites in Ni/MgO and Ni/TiO2. The spinel NiAl2O4 formed in Ni/gamma -Al2O3 might be responsible for its high activity and resistance to coking and sintering because it can produce a highly dispersed active phase and a large active surface area as bound-state Ni species when the catalyst is prepared at high calcined temperatures or with low nickel loading. The addition of La2O3 or MgO as alumina modifiers can also be beneficial for the performance of the Ni/gamma -Al2O3 catalyst.