The catalyst level of the ebullated-bed is an important process indicator in operating the II-oil hydrotreating unit in Shuaiba Refinery, Kuwait National Petroleum Company (KNPC). In an attempt to use mixed hydrotreating catalysts in one of the II-oil reactors, the catalyst bed expanded more than usual, and the level was very unstable. Moreover, a reduced unit performance and unstable operations were resulted, which led to the unit shut down and process time loss. In a joint study between the Petroleum Technology Department at Kuwait Institute for Scientific Research (KISR) and KNPC, the hydrodynamics of the mixed and individual catalysts in a cold flow ebullated-bed reactor (EBR) were found to be very different, though the two catalysts had similar physical properties. Further study revealed that catalyst attrition was a very important factor in contributing such reactor behaviours. (C) 2001 Published by Elsevier Science B.V.