When a Co/Mo catalyst is used for processing vanadium-containing heavy oils, vanadium deposits on the catalyst. As the amount of vanadium on the CoMo catalyst increases, the catalytic effects of CoMo decline and the presence of vanadium starts to influence the hydroprocessing products. Model feeds have been used to explore the changes in the catalytic activity of CoMo, aged CoMo, and VSxon alumina. Desulfurization, denitrogenation, deoxygenation, aromatics hydrogenation, and metals removal were monitored. Upon the addition of hydrogen sulfide to hydrogen, improvements in the catalysts for aromatics hydrogenation, denitrogenation and metals removal were observed.