A study is made of Mn//Mo, Fe//Mo, Co//Mo, Ni//Mo, Cu//Mo or Zn//Mo stacked beds systems, separated by g-Al2O3, in the gas-oil hydrodesulphurization reaction, under experimental conditions similar to those industry. The synergism between these beds is clearly detected, and occurs in the absence of mixed phases. The results show that the promotion effect of the pairs in stacked beds, caused by the spillover hydrogen increases when the liquid spatial velocity decreases.