Activities of supported MoS_X-based hydrotreating catalysts for several reactions as hydrodenitrogenation (pyridine), hydrogenation (pyridine, toluene, alkadienes) or isomerization (alkadiene) have been correlated with the number of coordinative unsaturations located in the neighborhood of the Mo species (^XM). These coordinative unsaturation sites (CUS) are controlled by a pretreatment under H₂ at various temperatures -T_R-. The hydrogenation reactions of aromatic compounds (toluene and pyridine) and the isomerization reaction of conjugated dienes need a ²M-²M or ²M-³M site. The possibility of hydrogenolysis of piperidine on the same sites makes up the pyridine HDN. The ³M-³M sites are responsible of the complete diene hydrogenation and of a pyridine HDN without piperidine desorption. The role of the promoter (Ni) has been also examined.