Ex situ and in situ decompositions of ammonium and tetraalkylammonium thiomolybdates were used to synthesize unsupported MoS 2 and CoMo catalysts. The ex situ activation was performed under a H 2 S/H 2 flow whereas the in situ activation consists in decomposing the thiosalt precursors in the presence of a hydrocarbon solvent during the hydrodesulfurization (HDS) of dibenzothiophene (DBT). Particular attention was devoted to the use of carbon-containing precursors and to the synergetic effect of cobalt. Ammonium thiomolybdate (ATM), tetramethylammonium thiomolybdate (TMATM), and tetrapropylammonium thiomolybdate (TPATM) and their cobalt-promoted counterparts were studied. Catalysts were characterized by XRD, scanning electron microscopy (SEM) and specific surface area measurements (BET, BJH). Catalysts were evaluated for the HDS of DBT. The in situ method of preparation allows to obtain catalysts with larger superficial areas than those obtained with the ex situ method. The cobalt synergetic effect depends both on the method of activation and the presence of carbon in the precursor. For in situ activated catalysts, the promotional effect of cobalt is well-observed if carbon is present in the precursor while an opposite effect is shown for ex situ activated catalysts.