The influence of the nature of the support on nickel catalysts behaving as methanation or Fischer-Tropsch catalysts is studied. This has been done by varying the support (silica and titania), the nickel loading (2-20 wt.%) and the reduction temperature (573-773 K). For in situ characterization of the supported nickel catalysts magnetic measurements have been performed. Silica-supported nickel catalysts show a very high selectivity toward methane, which does not depend on the nickel loading or the reduction temperature, Highly loaded (20 wt.%) titania-supported nickel catalysts which are reduced at relatively low temperatures, also show this methanation behavior, However, high-loaded catalysts reduced at high temperatures as well as low-loaded catalysts show the production of high amounts of higher hydrocarbons. The nature of the carbon present at the surface of the catalyst during Fischer-Tropsch synthesis is different for the catalysts which produce methane and for the catalysts which show the production of higher hydrocarbons. These differences between the two groups of catalysts are most probably due to an interaction between nickel and TiOx (x < 2) species, which can cover part of the nickel particles. The TiOx species promote the dissociation of CO and decrease the hydrogen dissociation rate. This change in dissociation properties can probably cause the change in the product selectivity and the different types of carbon present at the surface.