The effect of the catalyst precursor formulation on the hydrotreatment of Belayim atmospheric residue was investigated and different Q(2)[MoxSy] complexes were tested. The effect of the counterion Q and of the y/x ratio were examined; the most interesting results were obtained for S/Mo less than or equal to 4 with [Mo3S9](2-) as the best precursor and with phosphonium salts as the best counterions. Phosphorus promotion might be related to vanadium demetallation. These results were compared with those obtained by using MoO2(acac)(2) (acac = acetylacetonate) as the catalyst precursor. In order to discriminate between thermic and catalytic aspects, the effect of catalyst concentration was carefully examined; the higher the concentration, the higher the removal of heteroatoms from the residue without producing too many light products.