The effect of Co/Mo ratio in the CoMo/Al2O3 hydrotreating catalysts prepared by simultaneous using of the decamolybdodicobaltate heteropolyacid (Co(2)Mo(10)HPA) and cobalt citrate was investigated. Synthesized catalysts were characterized by Raman spectroscopy, N-2 adsorption, XRD, XPS, HRTEM techniques and were examined in the model reaction of 4,6-dimethyldibenzothiophene (4,6-DMDBT) HDS, in hydrotreating of straight-run gas oil (SRGO) and mixture of 80 vol.% SRGO with 20 vol.% light coker gas oil (LCGO). It was established that Co/Mo ratio in the catalysts significantly influences on the structure of CoMoS phase of type II and catalytic activity. Catalysts with lambda = Co/(Co + Mo) = 0.35 had maximal activities in 4,6-DMDBT HDS and diesel HDT and showed the best stability. Increase of the Co/Mo ratio led to simultaneous growing of both average length and stacking number of CoMoS multilayer active phase of the catalysts. Average length increased from 3.0 to 4.0 nm, average staking number increased from 1.4 to 1.6. It was found out that catalysts activity decreased within Co/Mo ratio lambda from 0.35 to 0.45. Analysis of textural properties of the catalysts, active phase morphology, phase composition and catalytic activities allowed us to suppose that with increase of Co/Mo ratio blocking the mesopores by sulfides of the active components and by coke species with formation of new micropores would take place. The activity of the best prepared CoMo/Al2O3 catalyst was competitive with the activity of the well-known commercial catalysts for ultra-low sulfur diesel in hydrotreating of SRGO or mixture of SRGO with LCGO. (C) 2011 Elsevier Ltd. All rights reserved.