The sulfidation of crystalline MoO3 and the thermal decomposition of (NH4)(2)MoO2S2 to MoS2 via an {MoOS2} oxysulfide intermediate have been studied by means of monochromatic X-ray photoelectron spectroscopy (XPS) and infrared emission spectroscopy (IRES). Several basic steps of the sulfidation reaction could be resolved and explained in terms of the structure of crystalline MoO3. The sulfidation reaction starts at low temperatures with an exchange of terminal O-2(-) ligands of the oxide for S2- by reaction with H2S from the sulfiding atmosphere. In subsequent Mo-S redox reactions, bridging S-2(2-) ligands and Mo5+ centers are formed. Lattice relaxation and further sulfur uptake are the main processes before, at temperatures above 200 degrees C, direct reactions with H-2 occur, during which the Mo5+ centers are converted into the 4+ oxidation state. The decomposition experiments with (NH4)(2)MoO2S2 show that terminal O2- ligands serve as the reactive sites, The Conversion of terminal Mo=O-t to Mo=S entities and the subsequent generation of Mo=O-t from mu(2) and mu(3) oxygen atoms in the MoO3 lattice appear as the general principle of the sulfidation reaction.