Aging tests of HDS catalyst in two-stage RDS process (HDM/HDS) were performed with Arabian light atmospheric residue as feedstock and the deactivation mechanism was discussed by comparison with previous results using Kuwait residue. It was found that the HDS catalyst in the second stage was deactivated by coke deposition rather than by metals and that asphaltene aromaticity in the feed was a good index for the HDS catalyst deactivation. Furthermore, it was confirmed that fouling rate was strongly affected by the combination of asphaltene aromaticity in HDS feed and reaction temperature in HDS itself. (C) 2001 Elsevier Science B.V. All rights reserved.