For H(x)NaY supported metal sulfide catalysts the intrinsic catalytic activity [thiophene hydrodesulfurization (HDS)] and the metal sulfide dispersion (dynamic oxygen chemisorption) are measured as a function of the acidity of the zeolite support (determined by ethylamine temperature-programmed desorption). The results show that the acidity can have a strong influence on both the conversion and the product selectivities. An increasing acidity results in an increase of the initial thiophene HDS activity. The increased activity is not caused by an increase in the metal sulfide dispersion, but, probably by a synergetic effect between the metal sulfide particles and the acidic zeolite support. The increase in initial thiophene HDS activity with increasing support acidity is also observed for cobalt, nickel and molybdenum sulfide catalysts prepared by impregnation using various zeolite supports (ZSM-5, Y, HUSY). The higher activity may be caused by an increase in thiophene adsorption in the zeolite pores, or by a direct effect of H+ on the thiophene HDS reaction. Additionally, the acidic supports themselves also show a considerable initial HDS activity, indicating that the acid sites are able to desulfurize thiophene at reaction conditions.