Hydroconversion of an atmospheric residue (AR) was performed in an autoclave with Mo, Ni, V compounds applied as precursors of the dispersed sulfide catalysts. The reaction was conducted at 450 degrees C, residence time 1 h and the catalyst concentration in the range of 300-900 ppm. AR conversions were compared for the reaction carried out in the presence of sole metals and for the mixtures of two metals. By mixing of the Ni and Mo precursor salts together, improvement in the total conversion and hydrodesulfurization (HDS) degree of the AR was observed, indicating on a significant synergy between Ni and Mo dispersed sulfides. By contrast, in the case of V and Mo precursors mixture the total activity was just a sum of the contributions from two respective metals. With the support provides by high resolution microscopy (HRTEM) and core-hole X-ray absorption spectroscopy (XAS) it was concluded that in the case of mixed Ni-Mo catalyst, MoS2 slabs were deposited on the surface of Ni3S2 crystallites, providing a reverse synergistic effect. (C) 2015 Elsevier Ltd. All rights reserved.