Gas-phase activation and liquid-phase activation were compared for a suite of CoMo/Al2O3 catalysts with phosphoric acid (PA) and/or citric acid (CA) as additives. Gas-phase activation was carried out in 1 or 20 bar H2/H2S(10%), whereas liquid-phase activation comprised sulfidation in a model diesel feed, spiked with dimethyldisulfide and 20 bar H2. Full sulfidation of calcined precursors was difficult to achieve (type I), as shown by XPS and EXAFS. CA addition slowed the sulfidation of cobalt and molybdenum in 1 bar H2/H2S compared to a catalyst with no additives, but it substantially increased sulfidation rates in 20 bar H2/H2S. This can be attributed to reduced support interactions and enhanced reducibility of molybdenum. Liquid-phase sulfidation was initiated by a release of H2S at ∼250 °C due to decomposition of the spiking agent dimethyldisulfide. The resulting active phase was composed of predominantly MoS2 single-layers, whereas stacked layers formed after gas-phase activation. Catalytic activity in thiophene and DBT HDS mainly depended on the number of promoted sites. Catalysts prepared with CA had the highest degree of promotion and were most active in thiophene and DBT HDS, whereas the catalyst with PA performed best in gas-oil HDS due to increased stability.