Sulfiding is a necessary preliminary treatment for the hydrotreating supported CoMo and NiMo catalysts. Sulfidations of two bulk molybdenum precursors, ammonium heptamolybdate and molybdenum acetylacetonate, with 5 wt % H2S/H-2 have been investigated as a function of temperatures. The course of the reactions was followed and studied by means of X-ray diffraction and gravimetric analysis. The sulfidation extent of bulk ammonium heptamolybdate was found to be highly dependent on temperature. Three main successive steps are suggested to be proceeded through sulfidation. These are calcination to MoO3, reduction to MoO2, and ultimately direct sulfidation to MoS2-2H structure. Results show that crystalline MoO2 is an intermediate in sulfidation of ammonium heptamolybdate. In addition, a fully sulfided phase to a highly crystalline MoS2-2H was obtained at ca. 800 degreesC. However, sulfidation of molybdenum acetylacetonate occurs quantitatively at ca. 400 degreesC and leads likely to the formation of a nanocrystalline single layer of MoS2. A sulfiding scheme is proposed.