The progressive sulphidation of molybdenum-based hydrotreating model catalysts is studied using HRTEM and XPS. The catalysts are sulphided quasi in situ and prepared for TEM measurements in a glovebox. Subsequently, a specially developed vacuum-transfer specimen holder is used for transfer to the TEM. Using this procedure, a previously unknown feature is observed in the catalysts. Apart from the well-known slab-like structure of bulk molybdenum disulphide, manometer-sized spherical entities of molybdenum sulphide or oxysulphide are observed. For a series of model CoMo/silica catalysts, these "spots" predominantly occur after mild sulphidation procedures. At higher sulphidation temperatures, these spots exist next to the slabs of MOS2-like material. Comparison with practical molybdenum-based hydrotreating catalysts shows that these, too, can contain spots next to or instead of slabs, depending on precursor preparation and sulphidation procedures.