A series of aluminum borates with various Al/B ratios were prepared by the precipitation method. These materials were used as the support of CoMo and NiMo catalysts. Hydrotreating of heavy Kuwait atmospheric residuum were carried out in a bench scale trickle bed reactor at 663 K and 7582 kPa. These catalysts were more active than the alumina-supported catalysts. An optimal boria content which gave the highest HDS and HDM activities was located around 4 wt% for both CoMo and NiMo catalysts. In addition, CoMo catalyst was always more active than NiMo catalyst provided the same boria content for both HDS and HDM reactions. (C) 1999 Elsevier Science B.V. All rights reserved.