Coke deposition on an industrial Mo/Al(2)O(3) hydrodemetallization catalyst and a Ni-Mo/Al(2)O(3) hydrodesulfufization catalyst was studied as a function of operating temperature during hydrotreating of Kuwait atmospheric residue. The primary objective of the study was to examine the nature of initial coke deposits and to examine the relation between solubility of asphaltenes in the product oil and coke deposition in residue hydroprocessing. The quality of coke on catalysts (hydrogen to carbon molar ratio) decreased with the quality of asphaltenes in terms of its aromatic character in the product oil, indicating that the coke property related to the asphaltenes property. A new index, the relative solubility index, was defined in this work. Quality and quantity of coke on the spent catalysts were found to be affected by the relative solubility index. (C) 2004 Elsevier Ltd. All rights reserved.