The effects of preimpregnation of alumina with ammonium fluoride, incorporation of nickel and full sulfidation of tungsten catalysts on the catalytic performance of two series of tungsten catalysts, one made from ammonium metatungstate and the other from ammonium tetrathiotungstate (ATT), were investigated. The catalysts made from tetrathiotungstate had a much higher activity than the ones made from metatungstate, and fluoride addition substantially increased the catalytic activity in the hydrodenitrogenation (HDN) of o-toluidine. The elementary reactions involved in the HDN of o-toluidine were affected to different extents by full sulfidation, the fluorination of the alumina support and the incorporation of nickel, proving that these reactions occur on different sites that were modified in different ways. The hydrogenation of the phenyl ring and the C(sp(2))-N bond breaking improved most, while the hydrogenation of the alkene and the C(sp(3))-N bond breaking were affected to a lesser extent.