Hydrodesulfurization (HDS) activity and activation energy (epsilon' (TH)) values were determined and compared with irreversible sulfur uptake (S-irr) by 5 Al2O3 supported (MoOx, Ni and three NiMoOx) catalysts and, in unsulfided form with the cyclohexane conversion activity. Synergy between Ni and Mo in catalytic activity and a correlation between HDS activity and the amounts of Sirr was found. Some explanations for the differences in catalytic behavior of the different samples are presented.