Abstract: This paper is concerned with the dispersive model on the surface of Ni-Mo catalysts. Although the catalysts were prepared with double impregnation, it was found by XPSthat active ingredients were well dispersed on the surface of the support a'nd did not cover each other. Amonolayer model was proposed instead of a "bilayer" structure suggested by some literatures.The HDNexperiments showed that activity of catalysts varied with the composition of catalysts and reached, a maximum at 3.5%(wt) NiO-14.5%(wt)MoO₃. The similar result was reported for HDSin some literatures and explained by the synergism-by-contact model proposed by Delmon et al. It is apparent that the result in this paper could not be explained by that model.