Gas-oil hydrodesulfurization activity of Ni-Mo/Al2O3 catalysts was significantly improved when prepared with zirconium alkoxide, but lowered when prepared with Zr nitrate. IR of CO adsorption on reduced catalysts, and LRS and XPS of calcined catalysts show that the addition or zirconium does not effect the metal oxide structure. IR of pyridine adsorption and Al-27 NMR both show greatly enhanced Lewis and Bronsted acidities of the Zr alkoxide modified materials as compared to unmodified samples. These results point to the strong influence of surface acidity on activity in hydroprocessing catalysts.