Genesis of alumina supported hydrotreating (HDT) catalysts prepared with the use of decamolybdodicobaltate heteropolyanion (Co(2)Mo(10)HPA) and cobalt citrate during their sulfidation processes and deactivation in diesel HDT has been investigated. The sulfidation stage was studied for two procedures: gas phase sulfidation by H2S/H-2 and liquid phase treatment by a mixture of dimethyldisulfide in diesel at various temperatures and durations. The catalysts have been studied by N-2 adsorption, thermogravimetric analysis, X-ray photoelectron spectroscopy and high-resolution transmission electron microscopy methods. The catalysts were tested in HDT of mixture of 70 wt.% straight run gas oil with 16 wt.% light cycle oil and 14wt.% light coker gas oil. Mechanisms of the active phase formation in the course of gas and liquid phase sulfidation processes have been established. It was found that gas phase sulfidation led to formation of the CoMoS active phase with higher cobalt content comparing to liquid sulfidation of the catalyst and initial activity of the gas phase treated catalysts in diesel HDT was also higher than catalysts subjected to liquid sulfidation. Catalytic examination after accelerated deactivation conditions showed that the liquid phase sulfided sample was more resistant to the deactivation. Probably it is due to stabilization of active phase particles by coke formed intensively during liquid phase sulfidation. The results were discussed using the recently developed concept of interlayer dynamics of the active sites of the CoMoS phase. (C) 2014 Elsevier B.V. All rights reserved.