Unsupported Co promoted MoS2 catalysts for the hydrodesulfurization (HDS) reaction were prepared using a mixture of Co(dien)(2)MoS4 (dien = diethylenetriamine) and tetrapropylammonium thiomolybdate. The mixture was first treated in a high energy ball mill followed either by ex situ activation in forming gas or by in situ activation during the HDS reaction. The ex situ activation mode leads to a very active catalyst exhibiting 84.9 % conversion of dibenzothiophene within 5 h. The in situ activated catalyst is less active but still shows a remarkable performance. Transmission microscopy investigation demonstrates that in the ex situ activated catalyst the stacking of MoS2 slabs is lower than in the in situ obtained material, which is corroborated by Raman measurements on the materials. In addition, the average MoS2 slab length of the former sample is significantly shorter than for the latter material. During the in situ activation, crystalline Co9S8 is formed which may be a reason for the lower HDS activity. The chemical composition of the two catalysts is also different, with larger carbon and sulfur contents found for the in situ-formed sample. The deposition of carbon and sulfur on the surface of the catalyst may also account for the lower catalytic activity, compared to the ex situ obtained material.