The addition of barium, as a secondary promoter, to alumina-supported Ni-Mo (nickel oxide-molybdenum oxide) catalyst was investigated. The results of thiophene hydrodesulfurization experiments carried out on the catalysts show that the addition of small amounts of barium (<0.7 wt%) leads to a significant improvement in their thiophene hydrodesulfurization. Experiments carried out in an autoclave also show that the presence of barium leads to improvement in the properties of the products obtained after gas oil processing. These results may help in developing catalysts with improved activities, as only a small amount of barium secondary promoter is required.