The catalytic behavior of a Ru (5%)/Al2O3 catalyst was studied in the straight run gas oil/light cyclic oil (SRGO/LCO) hydrotreatment at industrial conditions. Prior to the catalytic tests, the extrudates were sulfided at high pressure (P = 55 mPa), at two different sulfidation temperatures (503 and 573 K), with four different sulfiding mixtures (CH3)(2)S-2-naphta-N-2, (CH3)(2)S-2-naphta-H-2, CS2-naphta-N-2 and CS2-naphta-H-2. For comparison purposes, the Ru (5%)/Al2O3 catalyst was also sulfided using H2S (6 vol%)/N-2 (P = 11 MPa). Complementary atmospheric pressure thiophene HDS experiments were also conducted. TPR-S experiments of the catalyst sultided at industrial conditions and at atmospheric pressure were performed and correlated to the catalytic behavior. The results showed differences of 60% in HDS catalytic activity as a function of the sulfidation conditions used to activate the catalyst. The characterization suggests that the activity differences are associated with the stability of the RuS2 phase in reaction conditions. (c) 2005 Elsevier B.V. All rights reserved.