Monochromatic XPS spectra of the temperature-dependent sulfidation of MoO3/SiO2/Si(100) model catalysts are compared with spectra of Mo-S cluster compounds in particular with those of (NH4)(2)[Mo3S13].H2O and its thermal decomposition products, XPS is used to identify different states of sulfur and molybdenum occurring during sulfidation, The spectra show the presence of bridging disulfide ligands and of substantial amounts of Mo-V in an early stage of the sulfidation, These findings suggest that the initial reaction of the MoO3-type precursor with the H2S/H-2 atmosphere consists of two elementary steps, namely an O-S exchange followed by an Mo-S redox process.