In situ decomposition of ammonium and tetrabutylammonium thiomolybdate (ATM and TBATM) were used to synthesize unsupported MoS[subscript 2] catalysts, the in situ activation consisted of decomposing the thiomolybdate precursors in the presence of hydrocarbon solvent under H[subscript 2] pressure at 350degrees Celsius during the hydrodesulfurization (HDS) of dibenzothiophene (DBT) or 4,6-dimethydibenzothiophene (4,6-DMDBT) as sulfur model compounds. In situ generated MOS[subscript 2] catalysts were characterized by XRD, BET, EDX and SEM. Parameters affecting the HDS reaction were investigated: hydrogen pressure, reaction time, amount of precursor, types of precursor, cobalt and nickel promoters, and water addition. Inhibition of pyridine was also examined. The results showed that, cobalt and nickel addition enhanced HDS activity of both in situ generated MoS[subscript 2] catalysts from ATM and TBATM precursors. The use of ATM precursor with added water for in situ generation of MoS[subscript 2] catalyst can lead to 100% conversion of HDS of 4,6-DMDBT under 30 atm H[subscript 2] pressure at 350degrees celsius. Model reaction suggested that water addition led to a high surface area, highly active MoS[subscript 2] catalyst. The optimal mole ratio of H[subscript 2]O/ATM = 1200 performed the highest surface area (559 m[superscript 2]/g). In addition, the catalysts could reduce the sulfur contents of straight run gas oil (SRGO) from 6100 ppm to 3250 ppm and from 310 ppm to 100 ppm in light cycle oil (LCO).