The cracking, hydrocracking and hydrodesulfurization activities of gallium-aluminum mixed oxide catalysts, with fluoride added as promoter, were investigated. The addition of fluoride to the Ni-Mo catalyst supported on the mixed oxides leads to a significant enhancement in the cracking (no metals) and the hydrocracking (Ni-Mo) reactions of the catalysts, respectively. This enhancement of activity is attributed to the promotional effect of fluoride and the presence of gallium in the support, which leads to catalysts that are more resistant to deactivation. The results of the hydrodesulfurization experiments are not so encouraging. The activity of the catalysts was depressed by the addition of fluoride and with increasing gallium oxide content of the support.