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 WS2 crystallites but has little effect on the number of layers. On the other hand, the change of precursor salt significantly influences the stacking of WS2 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.