A series of commercial Co-Mo/Al2O3 catalysts, freshly sulfided as well as used in different trickle phase and gas phase reactions, has been characterized by using (scanning) transmission electron microscopy combined with energy dispersive X-ray analysis [(S)TEM-EDX)] and X-ray diffraction (XRD) analysis. While liquid phase sulfided Co-Mo/Al2O3 catalysts contain only MoS2 single layers, some MoS2 stacking is observed following H2S/H-2 sulfidation. Liquid phase sulfided catalysts as well as catalysts partly sulfided with liquid prior to H2S/H-2 sulfidation have higher activities than those sulfided only with H2S/H-2. MoS2 sintering and Co sulfide segregation are observed in catalysts after commercial use under severe conditions. Compared to what is typically found for Ni-Mo catalysts, metal-rich agglomerates in Co-Mo/Al2O3 catalysts contain more Mo while the Co sulfide crystals are less well developed.