The influence of the titania support and metal particle size on the performance of nickel-iron catalysts in the Fischer-Tropsch synthesis has been studied by varying the nickel-iron loading and, consequently, varying the nickel-iron particle size. Low-loaded titania-supported nickel-iron catalysts (2 wt%) turned out to be more selective towards higher hydrocarbons than high-loaded nickel-iron catalysts (20 wt%), which produce much more methane. From temperature-programmed hydrogenation experiments, magnetic measurements and Mossbauer spectroscopy it followed that different types of carbon are present on the catalysts depending on the metal loading. These types of carbon could be related to the selectivity in Fischer-Tropsch synthesis and to the deactivation of the catalysts with time on stream. These differences in catalytic behaviour are most likely due to the presence of TiOx species on the surface of the active phase, which species decrease the CO and H-2 adsorption properties of the catalyst, but increase the dissociation of CO.