Catalytic system which can be used in processes for the hydroconversion of heavy oils by means of hydrotreatment in slurry phase, characterized in that it comprises: a catalyst, having the function of hydrogenating agent, containing MoS2 or WS2 or mixtures thereof in lamellar form or an oil-soluble precursor thereof; a co-catalyst, having nanometric or micronic particle-sizes, selected from cracking and/or denitrogenation catalysts. The co-catalyst preferably consists of zeolites having small-sized crystals and with a low aggregation degree between the primary particles, and/or oxides or sulfides or precursors of sulfides of Ni and/or Co in a mixture with Mo and/or W.