The structure of sulfided Mo-catalysts and their oxidic precursors has been abundantly studied, but the genesis of the active phase has remained much less investigated. The sulfidation (in H2S/H-2 atmosphere) of a series of MoO3/SiO2 catalysts has been examined by means of temperature-programmed sulfidation, X-ray absorption fine structure, and transmission electron microscopy. The oxidic, oligomeric clusters in a 5.6 wt % MoO3/SiO2 catalyst are transformed into partly sulfided particles (MoOxSy) by O-S exchange at room temperature. A molybdenum sulfide species the structure of which resembles the MoS3 structure is formed during sulfidation at 423 K. The MoS2 phase is formed at temperatures between 523 and 573 K, depending on the dispersion of the initial MoO3 phase. The transition of MoS3 into MoS2 can be monitored by the evolution of HPS from the catalyst with a simultaneous consumption of H-2. The two-dimensional size of the MoS2 slabs can be derived from the EXAFS Mo-Mo coordination number by means of a theoretical model. TEM is required to elucidate the stacking height of the slabs.