A series of alumina supported Ni, Mo and Ni-Mo catalysts were prepared for investigating the interaction between Ni and Mo in terms of their structural properties and catalytic activities. The catalysts containing 0-15 wt% Ni and/or Mo were prepared by impregnation or coimpregnation and were tested for BET surface area and total pore volume. CO chemisorption was used for metal surface area measurements. The catalytic activities of the samples were tested by CO2 methanation. Selected Ni/Al2O3 and Ni-Mo/Al2O3 catalysts with Ni and Mo loadings in the 5-15 wt% range were also examined by TEM and SEM-EDX, The results show that Mo changes the structural properties and the methanation activities of the catalysts. TEM and SEM-EDX studies support the synergetic interaction between Ni sites and MoOx species and show that the nature of the Ni-Mo interaction changes with the level of Ni and Mo loading. At low Mo loadings, Mo species act as a diluent of the matrix, promoting Ni particle distribution. At higher Mo loadings, partial coverage of the nickel particles is observed especially on catalysts with high total metal loading. SEM backscattered composition images indicate the presence of molybdenum in the oxide form which means that the active sites are the nickel crystallites.