A series of Co-Mo catalysts with different Co and Mo loadings were prepared and exposed to a commercial HDS run for a year. From the activity tests using model compounds, the HDS activity of the fresh catalyst was found to increase up to 4.0 Mo atoms nm(-2) with increasing Co-Mo loading. However, no significant difference in the activity was observed in the used catalysts. TEM and EXAFS analyses revealed that the MoS2 stacks aggregated in the lateral direction but did not grow in the normal direction to the layers during the run.