Two different series of sulfided NiW catalysts supported on alumina modified with different amounts of fluoride, in the range 0.0–2.5 wt%, have been prepared by using two different tungsten precursor salts: ammonium metatungstate and ammonium tetrathiotungstate. Samples of both catalyst series have been examined by the use of high-resolution electron microscopy. For the oxisalt-prepared catalysts the results indicate that fluoride incorporation increases the size of WS 2 crystallites but has little effect on the number of layers. On the other hand, the change of precursor salt significantly influences the stacking of WS 2 crystallites without greatly affecting their size. The thiosalt method of preparation also leads to an excess of sulfur in the catalysts, which is distributed in a nonhomogeneous way.