Air exposure (passivation) and subsequent resulfidation caused a substantial increase in the thiophene hydrodesulfurization activity of sulfided Co-Mo/Al2O3 catalysts. Since no effect was observed for Mo/Al2O3 and Co/Al2O3 catalysts, the passivation effect must be related to the Co---Mo---S structure. EXAFS, Mssbauer emission spectroscopy, and transmission electron microscopy measurements showed that the passivation effect is neither related to a change in the MoS2 dispersion, nor to a change in number and nature of the Co sites. A Co-Mo/TiO2, a Co-Mo/C, and a Co-Mo/Al3O3 catalyst, which was prepared with the aid of nitrilotriacetic acid, hardly showed an increase in the catalytic activity after passivation. Because of this, and since the Mo-S coordination number for a Co-Mo/Al2O3 catalyst is increased by passivation, it is concluded that the activity increase originates from a decrease in Mo-support interactions. These interactions are broken when Co atoms migrate to Mo---O---Al bonds during a passivation and resulfidation treatment and assist in the sulfidation of these bonds. Removal of steric hindrance is held responsible for the activity increase. The effect of passivation on the catalytic activity decreased with increasing Co/Mo ratio, probably because at high Co loadings most Mo---O---Al bonds are already sulfided in the first sulfidation step. A maximum activity increase was obtained at a passivation temperature of 340 K. At lower temperatures the oxidation and migration of Co is too slow, whereas at higher temperatures new Mo---O---Al bonds are formed in the passivation treatment. Ni-Mo/Al2O3 catalysts showed the same behavior as Co-Mo/Al2O3 catalysts.