High-resolution electron microscopy (HREM) has been used to determine the morphology of MoS2 (and WS2) crystallites formed upon sulfidation of hydroprocessing catalysts which differ by (i) the loading of deposited molybdenum, (ii) the presence of Co or Ni promoters, (iii) the presence of additives such as P, F, Na, and Cs, and (iv) the nature of the support (gamma-Al2O3, aluminates, zirconia). In general, the basis of comparison was established between samples having the same Mo (W) coverage m atom nm-2. Based on the detection of more than 400 crystallites on several pictures of each sample, diagrams representing the length L of the elemental platelets as well as the number N of stacked layers have been obtained. Lengths higher than approximately 9.0 nm and stacking superior to five layers have never been observed. The mean LBAR and NBAR values are between 2.4-4.4 nm and 1.4-2.7 layers respectively. For selected samples, it appears that there exists a correlation between NBAR and LBAR; LBAR increases when NBAR decreases, which reflects modifications in the stabilization energy of the disulfide crystallites or in the MoS2-support interactions. On the other hand, we did not find evident correlation between the morphology changes and the variations of catalytic activity in toluene hydrogenation.